I think the premise of your question overlooks a pretty substantial issue:
It's actually kind of hard to craft magical items
Even when using the more flexible, less expensive rules provided by Xanathar's Guide to Everything, crafting Magical Items is prohibitively difficult for players, greatly reducing the risk that they could overpower their campaign by crafting tons of magic items. The only real way for players to abuse this system is with a complicit DM.
The cost of crafting a Rare quality item is 2000gp, plus 10 work-weeks of effort (XGE, pg. 128-129). Ten work-weeks is a substantial amount of time, nearly 3 months. Now, as DM, I make a substantial effort to provide Downtime moments for the campaign relatively frequently, but even by those standards, it would take a character probably 15-20 sessions to successfully craft a single Rare-quality Magic Item. Multiple Party Members (each with the requisite Proficiencies) would be able to collaborate to dramatically lower that time; but that also means they're not doing anything else during their downtime.
And on top of that, 2000gp is not a huge quantity of money for a level 6 party (at an average of 88gp per CR5-10 encounter, DMG pg. 133, it would take roughly 23 random encounters for the party to acquire that much gold), but it is enough that the character probably isn't getting that money without their party members forgoing their share of the adventuring rewards. And bear in mind that that cost is an abstraction: both the DMG and XGE suggest and encourage the idea that crafting Magic Items should involve acquiring rare components (through purchase or adventuring) that mean simply acquiring the gold is insufficient.
Xanathar's Guide to Everything has the following suggestion on how difficult it might be to acquire the necessary ingredients:
An item invariably requires an exotic material to complete it. This material can range from the skin of a yeti to a vial of water taken from a whirlpool on the Elemental Plane of Water. Finding that material should take place as part of an adventure.
The Magic Item Ingredients table suggests the challenge rating of a creature that the characters need to face to acquire the materials for an item. Note that facing a creature does not necessarily mean that the characters must collect items from its corpse. Rather, the creature might guard a location or a resource that the characters need access to.
|Magic Item Ingredients
If appropriate, pick a monster or a location that is a thematic fit for the item to be crafted. For example, creating mariner's armor might require the essence of a water weird. Crafting a staff of charming might require the cooperation of a specific arcanaloth, who will help only if the characters complete a task for it. Making a staff of power might hinge on acquiring a piece of an ancient stone that was once touched by the god of magic—a stone now guarded by a suspicious androsphinx.
—Crafting an Item, Xanathar's Guide to Everything, pg. 128
It should be quite obvious that a level 6 party can plausibly acquire items from a CR9-12 scenario, but it will be quite difficult and potentially deadly.
So the reality is that if a character has the money/time/ingredients to craft a good magic item, it's because the rest of the party is collaborating to make it happen, whether or not they're literally doing so as part of the creation process. And if so, I don't really see the problem: D&D is, first and foremost, about collaborative storytelling, and if the players are all working in concert towards a specific goal, well... Mission accomplished.
Magic Items require a Recipe
The other major block to player characters crafting their own Magic Items is the fact that items require a recipe to be successfully crafted. There's plenty of characters that would want to craft a bunch of Belt of Hill Giant's Strength to hand out to their fellow players, but they first need to research/learn the recipe for crafting such a belt, and as DM, you have control over how long it takes to even learn that recipe.
And then they successfully learn that recipe: great! If they want to spend the time to start mass producing that item, they're only limited by how much gold and downtime you give them (again within your control); but they can only make that one type of item, and if they want to make something else, they need to learn how to make those other items.
So there are definitely scenarios where players might be able to craft lots of magic items... all of the same kind, where there's no guarantee that doing so will represent a substantial power boost for every character. 21 Strength is nice, but the Sorcerer still doesn't have Greatsword proficiency, nor do they have a very high Armor Class, so they'll still get stomped if they try to fight in melee combat.
Crafting items that every single party member can benefit from means having recipes for every single kind of item, and you have all the power in the world in deciding how easy or hard it is to get those recipes.
On the other hand: Uncommon and Common items
Setting aside the recipe issue, it's worth mentioning that Uncommon and Common quality items are substantially easier to make, requiring only 2 and 1 workweeks of work respectively, only 200gp and 50gp respectively, and requiring ingredients from far easier encounters. Clearly, if a character wants to fabricate items at this rarity, they'll have a far easier time of it.
By the standards laid out previously, there is a genuine chance that characters could acquire a very high quantity of Uncommon or Common quality Magic Items by efficient use of Downtime.
But here's the trick:
Uncommon quality items.... just aren't that good.
At least not once you're level 6.
Most Uncommon quality items are +1 weapons or items that are of comparable power. A +1 weapon is nice, especially since it counters the resistances of particularly tough creatures, but in most scenarios, it just represents a roughly 10% improvement to overall damage output. That's good, but it's not "break the campaign" good. Same for items like the Bracers of Archery, which are a +2 to damage but with no increase for attack rolls, which again averages out to a roughly 10-15% damage improvement (assuming the character was already optimizing for damage). Again: very nice, but hardly something that will break the campaign.
Other items, like Adamantine Armor, represent a 5-10% reduction in damage, A Shield +1 or a Cloak of Protection represents another ~10% reduction, but again: not quite game-breaking.
And just so it doesn't go unstated: Common quality items barely factor into this discussion, because the effects they have on combat are often pretty minimal.
If a character stacks as many Uncommon-quality items as they can without surpassing Attunement limits, they can expect to have as much as a +30-50% damage output improvement or (because of attunement limits) +20-30% damage input reduction, which is quite substantial; but this implies that they perfectly roll every single item they need to perfectly optimize, AND that they're given way more than enough time and money to successfully craft or acquire said items, which by the standards I've set above (especially given the issue of finding recipes for various items), that's incredibly difficult to accomplish with a single character crafting magic items, especially since they would need to have tool proficiencies for each different category of magic item they wanted to craft.
All for a 50% improvement to damage output and a 30% reduction to damage input, which basically just puts them on par with a character a few levels higher than them.
Considerations like this are why, at my table, if characters are a whole tier ahead of where they need to be to craft certain kinds of magic items, I'm often willing to just let them buy the items they need (for an appropriately expensive quantity of gold, of course)—so at level 6, they're allowed to buy Common or Uncommon quality items; at level 11, they can buy Rare items, and so on. Because like I said: these items just aren't that powerful, and for the amount of gold that they would cost, I've not seen problems with characters becoming overpowered just from having access to items of this quality.
I do think you could run into problems if these same characters could instead arbitrarily equip themselves with lots of Rare-quality items, since those items do represent a quite substantial increase in power, but as established, it's far harder to mass-produce such items.
Concerns about oversaturation of Magic Items, especially when faced with a character that can craft them themselves, are valid, but they need to be tempered with the fact that as DM, you have a lot of control over how many items they can produce, or which items they can produce, hewing to the basic constraints set by the rules in the Dungeon Master's Guide and Xanathar's Guide to Everything.
A Forge Cleric gains proficiency in Smith's Tools, and from the sound of things, they probably have a few more Tool Proficiencies that allow them to create other kinds of Magic Items. That'll offer a substantial variety, but it'll hardly encompass the range of items that can be crafted, and given that there are 17 kinds of Artisan's Tools in the game, the 3 or 4 proficiencies they could plausibly have at level 6 simply will not span the whole gamut of items they could possibly want to make.
So facilitate the Cleric's attempts to learn Magic Item Recipes, and I think that if you follow the guidelines provided by these sourcebooks, you'll find that there's simply not much risk of the party becoming overpowered.