Heretic of The Seventh Circle
Could do, but like with the cavalier fighter, you can do really interesting things with primary (sub)class features that are harder to do with spells or fighting styles, and can build on those to combine into a very strong focus, or can be used to split focus between roles, etc.Spellbook was optional but a really good feat.
I think you left out "magically bond with your main weapon" (and/or "enchant you weapon yourself")
I could fluff this as coming form a lot of different power sources (wizardy-arcane, bardic-arcane, any sorcerer origin, most warlock patrons), although each would probably deserve a few specific abilities to reinforce the flavor - so there's your subclasses. I'd probably leave tanky/striker-y as build options from things like fighting style and spell selection.
edit: as for the bonded weapon, yes! That’s one of my favorite bits! I made it a key pillar of my 5e Swordmage.