Even if you book a flight for 90 days and can get the 30 day extension, the rules still say onward travel must be within the number of days you will be stamped in for, because an extension is never guaranteed. That's what the bus ticket is for, incase they ask you on entry. Or if your airline asks which is more often than immigration.
Your visa is not in your passport, therefore there is nothing to transfer. To enter Thailand you will need to use your old passport which matches the passport number on your visa, and then they will put your entry stamp into your new passport which is valid.
You can remain in Thailand until July 2026, as long as you don't get divorced. If you get divorced, you must leave Thailand as of the day the divorce is final. Your extension will be invalid that day.
Since you said you work remotely, you should be able to qualify for a DTV. This will require you to do a border bounce every 180 days, but you'll have a 5-year visa. You cannot get this from within Thailand though, so you'll need to go to a nearby country like Vietnam or Laos to apply there.
Just book a bus ticket out of Thailand, and then worry about your return ticket once you know. You can apply for your extension 30 days early at most immigration offices and 45 days early in Bangkok, Chiang Mai and Phuket. That should give you enough time to plan.