Welcome to the forum! As for the ax5, yeah it isn't the greatest tranny but I have seen them get used pretty hard. I lost a tooth on my cluster in mine, had to skip 3rd gear all the way home from a snow run.
As for making it stronger, I don't think there is a way, general maintenance will probably be the best thing to do. I don't know if rebuilding it would be worth it, the ax15 swap may be cheaper, I know you said not feasible. I found a used ax5 for $75 on craigslist from a v8 swap. Swapped that with my bad one, changed the oil, and it's been working great. That's my thoughts, hope it helps.