Perhaps I'm still not fully understanding the different terms, but all I was initially looking for is information on which (Chinese) brands/companies, outside of the ones I already contacted, sell framesets with choose-your-own colors and don't require branding/logos. B2B entities don't matter for obvious reasons. For whatever reasons, some people felt that understanding the term "OEM" was relevant.
Almost every brand/manufacturer with dedicated frame thread on this forum provides frames without paintjob: Velobuild, Hygge, Hongfu, Dengfu, Speeder, Lightbicycle, XM Carbon Speed, you name it.
Honestly, I have no idea what Speeder's relationship is with Tavelo and Adapt. I also have no idea what the scope is of what these companies does and if they operate their own factories or are more like Seka. Speeder appears to be a component brand at this point, with their forks showing up on other brands' bikes labeled as "Speeder" in spec sheets.
Speeder and Tavelo, if not the same direct seller of Adapt frames, are at least very closely related to Adapt. What differs them, is that Tavelo is substantially more expensive, yet the finish of the frame is worse, as was indicated by 'bike mechanic-engineers' youtube bubble, the tolerances are worse and you need to get a file in your hands and get the job done right. With Speeder there were zero (or very minor) complaints regarding the frame or finishing kit and I can confirm the frame has top end quality.
A large chunk of Tavelo price comes from marketing expenses (influencer coverage) and target audience (westerners, who can simply pay more than the rest) at the expense of frame quality. Same goes for Seka, Winspace etc. That being said, more 'visible' brands more often than not provide worse 'value for money', even though they market themselves as 'value for money'.