I’m about to begin HRT and since I am not able to get that through my country’s public healthcare, it’ll have to be either DIY, or something like GenderGP or Imago.

I’m uncomfortable with Imago being a US company, but if it otherwise makes much more sense than GenderGP, that might not be an unsurmountable issue.
There has been talk that neither of those give as much support, or as good quality support, as they once maybe did. For example somebody actually reading the laboratory results and possibly recommending changing a substance or dose if so required would be would be a nice thing. Being middle-aged and having had relatives die at around my age because of blood clots, I know that at least I should choose some other ways than injections for taking the estradiol. An ability to talk with a specialist about this kind of stuff would be appreciated.

So, what’s the practical difference between Imago and GenderGP? Also, is there also a third one?

The most important service would of course be the ability to get an EU prescription through them so that I can go to a local pharmacy and buy the hormones and blockers there.