If you intend on doing the extension yourself, at the jomtien immigration office, you will need to file a TM30 within 24 hours of arrival, some people have reported that they have been fined, when they went to the immigration office in jomtien, to do an extension, and there wasn't a TM30 already on file, if want to make it easier for yourself, when you have 7 days or less on your current stamp, ( jomtien won't do it before that) just go to an agent and they will file a TM30, and do the 30 day tourism extension for you, it's usually around ฿4000 everything included.
you just need to have a photo on your phone of your current visa, anyone saying carry your passport around, it's not them that has to deal with it, if it goes missing!! if you live in Thailand and your current visa is due to expire, you would need to return to your home country, to avoid being on overstay, and to get a new passport, at great expense and a lot of messing around you can do without.
people that live in Thailand, don't generally do withdrawals straight from their home country bank, most do a transfer into wise for instance, which in the UK is a local transfer, so it's better to just do that, when you need to move money to Thailand, also set up all your payees with small transactions, because when you come to Thailand and use your banking app, there will be no suspicious transactions because you might get blocked, just use your Thai bank for everyday usage.
I've heard people mention the exchange booths in the Arab district in Bangkok, I believe they are a lot less fussy with the quality of the notes, it's worth a try I suppose if you're in Bangkok, or just ask a fellow Brit that's on his holidays, if he would change it for you, I remember you posting a while back about having some health issues, if you don't mind me asking, how is your health now??
Even if you still have a couple of weeks on your current stamp, do a border bounce just before the expiry date of the 6th of September, that will get you the maximum time on that visa, depending on when you first enter Thailand you can usually get between 8 and 9 months out of an METV.
Have you seen the native farang on YouTube?? he has lived in the UK for 20 years, and has recently moved back to Thailand, his girlfriend is a British woman, I think he had to get a visa until everything was sorted out, he would probably give you advice if you made contact.
I've heard rumours of people saying it's hit and miss, with bus and train tickets, to prove onward travel, because they want to see a ticket that shows a destination in another country, any train or bus ticket is just a ticket to the border, it's not a ticket that transports you across the border like a flight does, that's why they seem to much prefer you to show a flight, that's my understanding of the situation anyway.
Forget them "fake" onward ticket sites, if you're worried just book a cheap flight ticket to a neighbouring country, that exits before the time finishes that you will be stamped in for, the best way is to just try to check in first, if you have a problem, just step away from the counter and do it there and then, and show it to the airline staff.