What follows is all IMHO, blah, blah, there is no wrong way to play, blah, blah, blah, why doesn't everyone just understand everything on the internet is just opinion without having to state it all the blah, blah, blah.
Fixed ability score magic items are just horrible, they invalidate character choices, they don't help the characters they should, just horrible.
Attunement slots don't mean anything at all, think about it, your character has three, every character in the party has three, magic items are supposed to be infrequent, not all magic items in fact it seems few require attunement. All of that adds up to not having worry about attunement slots until everyone in the party has found 3 items each that require it, what level does this happen?
I mean if you have 5 characters in the group that is not until around the 16th magic item requiring attunement do you need to worry about it, if one in three magic items needs attunement thats what 60+ magic items floating around the group?
So my solutions, make every non-expendable magic item give a basic bonus and something extra if attuned, that way what 3 items you choose to attune matter more, and the limit comes into play earlier on in the game, fixed ability score magic items won't exist in my game, instead items of increased attribute will either add a flat +2 bonus or give a short duration special ability like an amulet of health giving you regeneration for an encounter.