POTENTIAL IMPERFECT SOLUTION
After months of experimenting I learnt some funny things. I'm no computer expert, so this is just anecdotal experience.
Me - Intel MacBook Pro
Her - M1 MacBook Air
My issue:
When I host my girlfriend would crash when I go into the mines or the desert.
'Solution'???
We decided to let her host another farm. On that farm she obviously would never crash as the host. What was strange was that after I entered the mines in her farm, when we got back to me hosting my farm, she could enter the mines and have a relatively stable game. Crashes still did happen, but with much much less frequency. So the mines are stable, but the desert wasn't. I jumped back into her farm where she was hosting, and used SMAPI debug commands (I learnt the basics just for this) to warp to the dessert and skull cavern. Back to my farm with me hosting, and things seem stable.
Other issues I bumped into:
We seem to need SMAPI on both our games for us to connect. Haven't tested this much so I can't say more.