Casting scrolls with UMD is likely to fail
UMD lets you ignore "all class, race, and level requirements on the use of magic items", but don't forget there are often other requirements for magic items.
You mention spell scrolls, for example they have the following rules:
If the spell is on your class’s spell list, you can read the scroll and cast its spell without providing any material components. Otherwise, the scroll is unintelligible.
The first ruling the DM has to make is whether or not a spell being on a spell list counts as a "class" requirement. I would say it does count, but it's up to your DM of your table.
Sage Advice does say that "The intent is that a Thief can use spell scrolls with Use Magic Device", but whether or not you follow Sage Advice is up to your DM (I personally do not, I just stick to RAW!). This would have been a great opportunity for them to clarify whether it is intended for Thieves to have to roll to use scrolls, but unfortunately it isn't elaborated on.
If they do ok it, you still need to deal with the rest of the rules;
If the spell is on your class’s spell list but of a higher level than you can normally cast, you must make an ability check using your spellcasting ability to determine whether you cast it successfully. The DC equals 10 + the spell’s level. On a failed check, the spell disappears from the scroll with no other effect.
Since thieves have no spells on their list, nor spellcasting ability, they need to make this check to use any scroll. Spamming scrolls of fireball sounds cool, but you need to pass a DC13 every time, and you have a +0 modifier - only a 40% chance of success.
However, even this part is questionable as someone could argue that not having a spell list means you can skip that requirement. Or that the spell list requirement is indirectly a class and level requirement, that it can be skipped. Ask your DM how they rule.
You may never find a magic item you can bypass
So, scrolls are a little sketchy. How about magic items? Many magic items like Necklace of Fireballs or Wand of Polymorph don't have class/race/level requirements. The former has none, the latter requires a spellcaster to attune (again, a DM ruling is required).
There are items like Staff of Power or Holy Avengers which do have requirements you can bypass, but they definitely do not pop up in campaigns often. Unless your DM is benevolent and plentiful with their magic items, you may never see one.
To illustrate how few items fall into this category, I did a search for wands and staffs common to rare and counted up how many at each tier. No common items can be bypassed, 4 uncommon, 5 rare items - and 7 which are questionable. Even if one of these items are found, you are competing with actual spell casters as to who gets to use them.
Also remember you still have to attune the item, using one of your 3 attunement slots.
When it comes to spamming spells, spellcasters do it better
Thieves have great versatility, but actually spellcasters have their own spells so they don't need items. They can also use the same items themselves. If what you want is to spam spells, being a spellcaster will get you there a lot easier.