Being attuned to and having an Ioun Stone of Mastery float around your head will increase your proficiency bonus by +1, which will total to +2 because of Expertise.
You also aren’t capped at 20 in an ability score, A Manual of Quickness of Action, the Star card from a Deck of Many Things, and The Book of Vile Darkness allow you to boost Dexterity by 2 and can exceed 20. Now as such you can get Dex to 30 which is a +10 modifier compared to a 20 Dex giving +5 but you’ll need to be one hell of a thief to steal that many of these items.
Lastly the boons on DMG 232. The Boon of Luck allows you to add 1d10 to an ability check, attack roll, or saving throw once per long rest. And if someone else has the Boon of Fate they can add 1d10 to your ability check aswell
Since the OP asked for things that may increase the total my numbers are all in addition to the numbers he came up with. So a list of effects I have suggested to increase the ability check result:
Dex 30: Adds +5
Ioun Stone of Mastery: Adds +2
Boon of Luck: Adds +1 to +10
Boon of Fate: Adds +1 to +10 but since this is an ability someone else must have my final answer has two sets of numbers since this isn’t something this character would be able to do by themselves.
So adding these numbers to OP’s numbers results in a minimum modifier (on their own) of +37 with a maximum modifier (on their own) of +64. Once more should you have a companion with the Boon of Fate they can increase the minimum modifier to +38 and the maximum modifier of +74. Factoring in the roll of a 20 on a d20 this results in a final maximum ability check of 84 or 94
Hopefully this is the last edit I make, but I would like to add my answer only covers bonuses from the PHB and DMG so if one of the published campaigns contains a magic item that can boost it further then I do not know about it. I can however mention that the character is attuned to a Stone of Good Luck, and an Ioun Stone of Mastery so this character is able to attune to one more item if one exists (Gloves of Thievery don’t require attunement).