Should I apply for the Soft Power visa or the Workation visa, given my tax situation?

I didn’t have to provide a tax return to the UK embassy. Just proof that I work remotely and had the 500k

If you do not meet the tax threshold then you let them know that and provide the prove you have. There are valid reasons for having no tax return and London approves many who do not submit them.

Apply soft power no problem from London.

Regarding the requirement for DTV, more embassies are now requesting it, as they want to ensure that individuals on Soft Power can sustain themselves while in Thailand and avoid engaging in illegal activities. As well, you already have some history what could be questioned. Some embassies may not require it yet, but this could change, especially when later doing visa extensions.

For taxes in Thailand, you'll need to check your tax status, especially if you spend more than 180 days per calendar year here and remit income from your online business to Thailand (which is likely). There may be a tax treaty, but the terms could favor Thailand. If your goal is to avoid taxes entirely while living in Thailand, this could be an issue. Tax liability applies to all individuals, regardless of visa type.