From our experience, once you start thinking seriously about SSP development, it stops being just a “let’s rebrand someone’s panel” thing and turns into a real product decision. We were in that spot a while ago and ended up talking to teams that specialize in this, including
https://geomotiv.com/industries/adtech/ssp/. What helped was that they look at the whole flow: integrations with demand, reporting needs for publishers, load, and support, instead of just slapping a logo on top. It’s definitely not a weekend project, but if you already have a network of partners, having your own branded environment with the rules you define can pay off in the long run.