Thailand is better but more expensive than Cambodia. If you could afford to retire comfortably in the West today, you will be able to afford to retire well in Thailand. If not, and assuming you plan to live for another 20+ years, you might get caught in the trap that many retirees to developing countries fall into of not accounting for income and price convergence over time - Cambodia will be relatively poorer, and therefore cheaper, for longer, so if you are relying on some kind of affordability arbitrage to make your pension go further than it would back home, Cambodia might be a better option.
Kids can get education visas with 1 guardian visa each until age 20. If you will be working remotely you will legally require a work permit and business visa (if planning to ignore this law, I recommend not to post about it online). If you are so rich you don't need to work you can get buy an Elite visa (after guardian visa).
I'm sure some rental companies would be happy to let you rent a car from them and drive it out of Thailand, as long as you leave a cash deposit equivalent to the full value of the car. Getting a car you don't own across international borders won't be as easy.
don't think you can do that from outside thailand unless you have the paper from the labour department to show you've already applied for the work permit.
If you are in America, chase the Thai embassy to get your COE today. If you are anywhere else, it's probably too late because they will now be closed for the weekend.
Western expats are not allowed to do farming or basically any blue collar work. You can pay a local a few hundred quid/dollars per month to mind your partner's farm and move her to Bangkok with you (you can probably also find work in Bangkok if you have significant professional experience in a particular field).