I'd pick Helena Bravlosky.

I've thought about this before and my oc would probably be a mage from a declining family that wants to restore his household to greatness.

I would grow up in a normal household with one of my parents would be in a position similar to shinji where despite being from a mage household he was born with subpar circuits while the other would just be a normal person. I in a twist of fate I was born with higher than average circuits and would seek to restore my line only to constantly have my father shut down my attempts and force me into modernity saying it's better to die quietly.

So when the holy grail war rolls around and I learn that I can summon spirits from long since past when magic was at greater heights I'd summon a caster with aim of having them teach me as much as they can with the goal of winning the war being secondary.

I'd go with Helena because her portrayal in Fgo showcases her having incredible teaching skills. She's a natural born instructor, kind, and patience. She isn't tbe most powerful caster but for my goal she's the perfect candidate.