How can I manage my entry and stay in Thailand for 9 months spanning two visits in 2024-2025?

In both cases you're eligible for retirement visa. Also you are entitled to retirement visa and dependent visas (this maybe useful because retirement visa has some requirements about proof of income). A retirement visa is valid for 90 days but can be extended for 1 year. There are some requirements to the retirement visa and some paperwork. The requirements vary by passport and if you are applying in-country or out of country. So look at both of those options.

Also, you can get a 60 tourist visa which can be extended 30 days given you a total of 90 days. You can do border bounce which is probably 30 days with a extension of 30 days for a total of 60 days. (I am not sure because I am more familiar with US national visa options which may be a little different for Aussies and Japanese).

I have applied for and gotten my eVisa while not in my home country. There is a lot a debate if it's possible or legal or whatever. From personal experience, its possible to get an eVisa while travelling from Thai Embassy USA, maybe the is the case for Aussie/Japan. If you can, then you can have a eVisa for your second leg of your journey which gets you 90 days.

60 day TV + 30 day extension - no problem. Another country? U are guessing which one? You don’t know?

The six months coming from another country (other than your own) is a bigger problem. But TV +30 + border bounce + 30 extension + border bounce gets it done

To get 90 days, must apply for a tourist visa on line in your country. 60 days (200$ fee) plus one 30 day extension @1,900$ baht nearest immigration. Or 30day+ one extention plus border run for 30 day plus another fee on both sides. Not easy, must be very careful and picky where you cross the border. There are agents that take you by bus with others and do it for you. Go to consulate website for the exact instructions. Border runs of course excluded.

That's not possible. Good that you checked. Not only is there a soft limit of staying on such entries, making that you can't plan as you now intended, there is also no 90 day tourist visa that you can extend by another 90.

The visa that you need in your case would be non-O or OA based on retirement. Probably best to take one out before your initial stay of 3 months. Then extend and get reentry permit to support the latter 6 months. Alternatively, you could get a tourist visa for 60 days, extend by 30, and then use the said visa only for the latter 6 months. There are two disadvantages: the initial period is now 90 days (or 89 rather as entry and exit day both count in full), being slightly short of 3 months, and you need to buy one extra extension. The advantage however is that you keep some period of your extended retirement visa alive, so you could easily return in the months after the initial 6.

You could also look into getting a non-OA visa. But I don't think you could both get one since you normally have to get it in your home country. But you might be able to have the one of you that is in your home country (not sure if you're in Australia or Japan now) apply for the non-OA and the other spouse apple for the non-O trailing spouse visa. The spouse on the non-O would need to go to immigration and apply for a 1-year extension based on the spouse with the OA before you leave the first time though.

A non-OA is a 1-year visa and you use money in your home country to get it, rather than showing money in a Thai bank account. It does have a high insurance requirement though.

For the first stay come in on tourist visas, gives you 60 + 30 days

For the second stay I'd say get a multi entry tourist visa which can give you close to 9 months if used judicially

The 90 day visa, by which I assume a non-o based on being over 50, can only be extended for 12 months by meeting the requirements (money in a Thai bank etc)

There's no visa that you can apply for a 90 day extension on. You can probably get away with entering visa exempt and then using a land border bounce agency for 2 additional border bounces, which should get you to about 6 months. That's 30+30, 30+30, 30+30. You would use the land border bounce agencies since you may run into issues trying to leave and return on your own after spending so much time in Thailand without a long term visa.