I have similar to you. Medication isn't too expensive and readily available without prescription. I have an annual scan of my carotids and its about 9000 baht including blood tests. No point in insurance for what I already have and that is being managed, so I've got a big wedge put aside should anything else pop up.
From outside of Thailand apply for a Non - Imm O visa. This will give you 90 days in Thailand to sort yourself out before going for your Extension based on retirement. Number 1 job on the list when you arrive is to open a bank account and deposit your 800k baht. Do not delay opening the bank account as you need to start seasoning your deposit.