I think I might have solved it. It's one of two things, and while I'm not certain which it is, I'm doing both currently so fingers crossed I'll be able to sort it out.
1. I have it full screen and I tab out a lot. At one point on a different legacy I was playing it crashed when I tried to tab back in, and I noticed that Moonlight Falls is laggier than the other towns, and possibly was lagging more whenever I tabbed out then back in.
1a. I have a game in Lunar Lakes, which is a smaller town than most with less buildings in it, and that has little to no lag. Could be my laptop not being quite up to spec.
2. The riverview game I was saving every two to three days. The new game I made to test Moonlight Falls I saved every 2 days, and it's got to week 2 now without crashing on saving.
So my guess is that Moonlight Falls is laggier than most on my computer due to being very large, my tendency to tab in and out of the game all the time was causing issues, and I wasn't saving often enough. Not sure why the latter might be causing the crashes, but it seems to have sorted it.
So probably would be best to move this to technical issues... While the specific town is playing a large part in this, it's more technical rather than town related.
Next step for me - finding a nice small sized map town to play on!
Edit: Spoke too soon. Did the same crash on my Lunar Lakes game on the morning of Thursday week 1. No idea what's going on now. The cursor continues doing its loading spin, but the middle icon saying "Saving" freezes, and then I have to Ctrl+Alt+Del to get the task manager up to quit out (can't even do Ctrl+Shift+Esc). Going to wait until I have better internet and try reinstalling everything sims related.