I have had both non imm oa and o extension based on retirement ...overall, particularly if you already have 800k in a thai account I found the o retirement extension option both easier to get and overall cheaper due to less certificates and insurances. Particularly as you pass age 65 and insurance premiums rise steeply often.
currently most land borders are closed or restricted. Neighbouring countries are opening up a little but the 'almost same day' border bounces are still not possible I think ...paperwork for entry and exit and re entry to thailand requirements blabla
Tod is right, altho I often take insurance for the expected length of stay and even higher coverage ...cant be bothered with multiple policies to cover medical eventualities.
You need to say what types of visa you are entering on. If a 30 day visa exempt or 60 day tourist visa, each can be extended at an immigration office for a further 30 days to give 30+30 or 60+30