I'm gonna try to shy away from the CR and purely mechanical aspect of balancing encounters and maybe tackle different aspects of what I read between the lines in this question.
As I understand, here are parts of your problem
- Fireball is used so effectively that you felt the need to nerf it. You're not the only GM to have this issue and I do not remember a GM ever nerfing that spell in my groups. Maybe there are alternative solutions ?
- Polymorph is also mentioned and I'm guessing you are having issues with what the wizard is turning itself into being possibly OP ?
- Cookie-cutter encounters are just that. You could build more challenging encounters by building them yourself while keeping your group's capacities in mind.
- If anything, from the list of magical item you mention, they are under powered for their level !! I'll try to tackle why they can still seem over powered while they mechanically are not, in my 'barely informed' opinion.
Fireball is about more than doing damage
It is an explosion. Fireball is not always safe to cast. It can backfire, too.
Fireball is a very popular spell. I don't have real statistics, but I'm sure 95% of players who can take it do take it. I've learned to deal with this spell when it gets abused and becomes the de-facto answer to any and all challenges I throw at my players.
Maybe casting a fireball inside of ruins is not safe ? It can cause passageways and roofs to collapse, possibly even ending the encounter before it starts by trapping the group, possibly even splitting them up ? For the sake of "fun" and to allow the dices to tell their story, I would not make this something that would automatically happen, rather give your spell caster a chance to notice that liberal uses of fireball might have consequences, besides efficiently killing mobs. If he keeps casting fireball, make the chances of a structural collapse more likely every time, until the group pays the price !
They are not in ruins or mines or caves than can collapse on their heads ? They might be in town where one fireball can set fire to an entire medieval city ? Or in a forest where carelessly burning trees might gather some most unwanted attention from the guardians of the forest ?
Fireball can also destroy items. That's also loot getting lost ! After 1-2 such occurences, I guarantee your other players will be the one trying to keep the wizard from using his fireballs.
You shouldn't be too heavy handed since what you want to achieve is for your spellcaster to be more careful, not have him stop using a spell that is legit.
Also, creatures with resistance to fire laugh at fireballs. Or your mobs might be buffed by spells that allow them to resist fire.
TL;DR Nerfing the spell is too heavy handed to my liking. Instead, I would make sure my players realize their are consequences to causing explosions. Also, there are spells and resistances that can be used to give an edge against fire damage.
Any problems with Polymorph ?
I'm gonna take a wild guess and assume your Polymorpher transforms himself into a T-Rex or some other very powerful but also very rare creature ?
As a GM, I often tackled this issue before it even could become a problem, with characters that I know will get access to that spell or any other kind of shape shifting abilities.
I make sure to make it clear to them that they do not know and have not encountered all creatures that exist in the world. I specifically rule the T-Rex out of the possibilities, unless the character can really convince me that it would make sense with a good story.
I'm actually pretty permissive with polymorph, most of the time only requiring the character to succeed on a Nature check to see if I allow him to transform into this form. I often adjust the DC of the check according to the CR of the creature. But then again, it has to be believable ...
I've had players able to transform into a T-Rex at lvl 7, as soon as they could. This is OP ... but Polymorph is also a Concentration spell, which is supposed to balance the power of the spell, so do not forget about having him roll concentration every time he gets hit ... the next time he loses concentration as a T-Rex (or any other powerful beast) and ends up as a wizard in melee range of a lot of enemies that absolutely will proceed to focus him (scared that he would transform into a scary monster again) I figure that's got to make him more careful in the future ?
Cookie cutter might not cut it
I also use random encounter generators, but only for inspiration. Most random encounters generators give you stat blocks, but an encounter is more than just a bunch of creatures fighting.
All random encounters happen somewhere. A lot of times, that somewhere is a place that is unknown to the group (they're just passing through, they don't live here !) but very well known to the mobs you want to put on your group's path.
(Note: All of the following suggestions should be appended with the condition "If that makes sense". I wont repeat it for brevity's sake).
Why wouldn't you add a few traps that the mobs would use to soften up any one unfortunate enough to pass through their territory ?
Maybe the creatures have some means to turn themselves invisible and instead of engaging the group in a head-on, fair fight, they would instead use guerilla warfare tactics to hit the group from multiple positions, ensuring that they cannot efficiently focus everything. Have your mobs disengage as soon as they can after being engaged so that they can kite your players.
If you play your cards right, aka if your mobs have some kind of advantage (preparation, terrain, numbers, etc.) even low level mobs can become a problem for a high level group.
How can my group be under powered on paper, but still breeze through all encounters that are supposed to challenge them, according to the CR mechanic ?
Like I said above, the list of magical items you showed us really is under powered for a level 7-11 group !!! At least, if that's all they have, but you said it yourself that you don't think you gave them too much and I readily agree with you.
So how is it possible that a group that is supposed to be under powered according to the DMG be over powered in reality ? At level 11, each character should normally have more than what you describe (see page 38 of the DMG for a nice little table, even in a low fantasy setting). Disclaimer: keep in mind I don't know the real extent of your group's wealth.
Having said that, I'm thinking the issue here isn't about the spells or magic items they use.
Maybe your players are just very good tactical minds and their characters also are, giving them such an edge that they aren't challenged ? Maybe they 'meta game' because they know all weaknesses of all creatures in the Monster Manual and other source books ?
There are a lot of mechanics you can use to introduce more challenges in your encounters.
Surprise might not work since they might be "that good".
Or maybe you can force them to make knowledge checks (Nature, Arcana and Religion all can give some insights to different types of monsters) before using OOC knowledge to give themselves an edge in those encounters ?
Worst comes to worst, boost some of your mobs, simply.
Victory is fun. But easy victories get old pretty quickly for a lot of people. I personally get bored if I never have to think outside the box to come up with solutions to challenges, so if all I need to do is roll dice to succeed I end up yawning and thinking about other funnier games I had in the past. That's just me, but I figure there are good chances you have players in your group that would agree with me on this.
What to do then ? Well, as a GM I often boosted my mobs on the fly. You don't need to give them access to fireball or polymorph, but a +2 to AC and/or to attack rolls can make a big difference in the damages a group will take even when they win.
Then again, you can surprise them with more than one encounter in a row. There are ways for you to narratively ensure your group cannot take a short or long rest before their next fight. Maybe that first encounter was easy because the mobs were not prepared for a group of high level PCs, but now that they know and have been alerted they might become a danger to the group's survival.
In conclusion: I hope you will find some of this advice useful. I personally constantly use those, and more, to keep my players on their toes. I make sure that they realize they are not fighting "stats blocks", they are fighting monsters who might sometimes surprise them with their behaviours and tactics...