The non-o IS the retirement visa. Get it from your Thai embassy before you leave the UK. Then when you arrive in Thailand, get your bank account opened ASAP because you need to transfer your 800,000 baht in there so that it has enough time to sit for 2 full months before you apply for your extension. That means you only have a couple of weeks to get it done if you want to apply before the last possible minute.
The workcation category is much less straightforward than soft power, but how difficult it is will be completely dependent upon your Thai embassy. They each set their own requirements.
Your visa type has nothing to do with bringing an animal to Thailand. Anyone can do that as long as they meet the requirements.
You can only apply for a multiple entry tourist visa in your home country or a country you have permanent residence in. So unless you are going back home, you won't be able to apply for a METV.
Also, it's a tourist visa. Immigration does not look kindly on people trying to stay long-term in Thailand without a long-term visa. There have been multiple reports of people with a METV getting grilled at immigration after leaving and returning immediately to get a new stamp, especially after doing it a second time.
If you applied for a tourist visa and it asked for a ton of stuff, then that means you applied for the multiple entry tourist visa, which you did not need for an 89 day stay. You could have applied for the single entry tourist visa, which barely has any requirements at all. But you don't need any visa, you can get a 60 day visa exempt entry, and apply for a 30-day extension, the exact same as a tourist visa.
TDAC has nothing to do with visas or your stay in Thailand. It's just an entry card required by everyone entering Thailand.
This is all but impossible now with the banking changes, because tourists cannot open a bank account, and showing money in a Thai bank account is a requirement for converting to the non-O visa.
Most embassies do not require a pension. They just require proof of income or proof of the 800,000 baht (or whatever the embassy's equivalent in local currency is) in a bank account.