That sounds incredibly frustrating, in my coding I just sort of... removed situations where third person pronouns are needed because I am a coward.
I'll try my best to do each context
[He] wants that - [They] want that
That doesn't belong to [him] - That doesn't belong to [them]
[His] friends are over there - [Their] friends are over there
Jess thinks highly of [himself] - Jess thinks highly of [themselves]
[He's] not very chatty - [They're] not very chatty (Present + Future tense)
[He's] done it - [They've] done it (Past tense)
I'm fairly sure this is all correct and don't think I've left out any context, but English is hard so I could be wrong. Hope this helped and good luck with the project, can't wait to see it finished