I'm confident I can do the work myself, I would prefer to so I learn something from the process even if it'll take a lot longer than if a pro did it.
I've come across something else though; would it be best practice to gear down the motor with a belt drive and adding a thrust bearing to the motor?
You really have to have a thrust bearing in there. An electric motor isn't designed to take thrust as a rule.
If you get serious about it I can find the reference to the SKF bearing & housing I used. It's a bearing that handles both thrust and axial loads. Makes a nice setup using standard industrial stuff. ATM I'm 1000 km away from my boat and reference material though.
What you're asking about is quite do-able if you want a project and experiment with it, so you're prepared to put in the time and keep tweaking stuff. I'd not do it myself given there's a functional engine but hey, I'm not averse to going down rabbit holes either if I feel like it.
Could be a slipping transmission too, now I think on it. In your position the first thing I'd do is sort out what's going on with the current setup, then decide.