I might be thinking of 3.5, but isn't there a risk of AoO when you use a Spell when engaged?

Not gonna lie, "magic stealer" is a really bad name. What's wrong with the classic spellthief?

It's cool that Vestige warlock can get Spirit Guardians via War domain spells, and the Vestige itself seems pretty big on HP. Can take pact of the blade to be a melee bruiser, with the vestige backing you up from range. I haven't checked to see if there is any other way to get both SG and Extra Attack by level 5 using only 2024 core-and-expanded rules yet, but this is an exciting development!