If you can prove you need to visit the UK regularly over a longer period, you can apply for a visa that lasts 2, 5 or 10 years. You can stay for a maximum of 6 months on each visit.
I have read reports that this can now be done at other immigration offices (e.g Jomtien and Chiang MaI although the application still has to be approved by BKK central office)
have you done this? my understanding was that the conversion to non-imm and extension of stay based on retirement was done at the same time (the first trip to immigration to get the apply for the conversion, and the second trip to get the extension based on the conversion)
just to clarify, you can use a mix of income and funds (should all add up to 800k). So if your bank funds were only 600k, you would need income of at least 200k per annum to cover. It isn't an either funds or income situation.
This can be done in Thailand though without any need to go for new non-imm in a neighbouring country. You will need sufficient remaining time on your TV though
Most banks have specific accounts for foreigners to use for this purpose which maybe you should look into for future years. Generally speaking, it is recommend that this account be separate from anything else and not used for anything. Most people end up in a mess because they have not followed this simple rule. It is a pain to have this amount locked up in an account, but there we are. You can hold the amount in Aus$ in a Thai bank account (BKK offer foreign currency accounts for example), but you may need to fiddle with the amounts if there are significant currency variations. It's better to just have the account in THB and dedicated for that purpose. If you need two accounts (I don't know the dependent rules), it would be better to have an account each rather than a joint account