When should I visit the local amphur to issue the Kor Ror 22 for my marriage visa before its expiry?

avoid using the word "work" or "job" or "employment" ...

First, if you want to apply for Soft Power, don’t mention anything about working, you will just confuse the consulate. You can apply already for the visa and enter directly with DTV during your vacation. Until January they might reduce the visa exemption to 30 days so you might not be able to stay the whole 7 weeks without extension, so it’s better to enter already with DTV. I’m sure you can start with Muay Thai in March, the schools are flexible

Take the vacation before applying as you need to show proof of your steady income for remote work. If you get caught doing remote work while on a soft power DTV it will get revoked as it only allows you to be in country for the course .... you need to be very clear with what you are doing in your paperwork for DTV

Don’t tell immigration you’re working when you’re supposed to be in the country for Muay Thai and vice versa. Their brain is wired to scrutinize any unambiguous person coming into the country. It’s not against the rules to do Muay Thai on workcation or do workcation on Muay Thai. Just keep it simple for yourself and Thai immigration 1 reason and stick to it. Because there is technically no written rules about DTV as far as I can find, the only written in stone rules are meeting the requirements to be approved for the application.

It seems that the soft power visas are being scrutinized after the course duration has expired. I'd just get the remote.

Yes keep it simple

Would just pick one route and keep it simple, either apply as a remote worker once you have the job, or do Soft Power through an eligible Muay Thai gym and follow their paperwork/process. No need to mix the two.