First of all, 20 is auto sucess ... no matter what modifiers you have ... in this game.
Second, well ...
You want to start with Astarion, prefferably at level 5 ...
- that should allow you get your dexterity on 18 ... meaning +4
- also level 5 incerase your proficiency bonus ... meaning +3
- also you have expertise ... meaning another +3
Then you want him to be fed ... meaning +1
Then you want him to wear
Smuggler's Ring ... meaning +2
And it certainly help when you have Guidance on yourself ... meaning +1d4
If guidance can be provided by another character than Shadowheart (wich can be done by using
Silver Pendant amulet), its also not a bad idea to give yourself Cat's Grace ... meaning advantage on Dex rolls.
And i believe that is all you can get ...
IN THEORY, you should be able to also add Bardic Inspiration for another +1d6 ... but it never worked to me with Guidance.
Anyway, if i count corectly that should give you ... +14-17 with Advantage ...
+15-23 with Advantage if you manage to sqeeze Bardic Inspiration there.
//Edit:
Oh and remember, no matter what modifiers you have ... 1 is auto failure.
So, maybe hold on some inspiration just in case.
