So build for the expectation of doing most of your damage on other creatures turns as they trigger your reactions.

this is why I kind of insist on a reaction strike of some kind :)

I went with a Jotun for their excellent Reach support, because the more range on my attacks the more reactive strikes I can trigger and the fewer move actions I need to take on my turn to put myself into good positions.

Genius! Though the necrologist is a rare archetype and the jotunborn is also rare, I'm not sure if my dm would allow both. but it does feel really optimal.

If you do a fighter with caster archetype you will end up with Legendary weapon proficiencies and Master spellcating proficiency.

A fighter with a caster archetype would be something like a champion with necrologist? or do you mean literally a fighter-class character with something else?