Welcome @PastorOfMuppets to the forum. Do you have the HD 6XX right now? What’s your current system (DAC and amp)? I ask because this information will help us to answer your questions with better advice.
You’re entirely right: the usual recommendation is to invest, first and foremost, in the transducer, then the amp, next the DAC, and eventually, once you’ve ascended to a certain, high level, the source. The HD 6XX/650 is tricky, though, because it scales so very much: an HD 6XX out of a totl amp is supposed to sound glorious and can be endgame for some (I’ve not heard such a setup but I can attest to the radical improvement of these headphones with superior amps). The upgrade path for the HD 6XX thus gets complicated by amp and DAC pairings. The Clear, by contrast, don’t necessarily scale as much, but they arguably perform better out of humbler setups.
I’ll defer to others with more experience: the Bifrost 2 is very highly regarded, but my understanding is that it’s slightly on the warm side of neutral, and I’m not sure if pairing it with a warm headphone like the HD 6XX would be an ideal match, although this will, of course, be determined by which amp you’ll be using. The overall system synergy is what matters.
I think you’re on the right lines in considering the Clear and Aeolus to be upgrades to the HD 6XX.
For planars, you might consider the Audeze offerings, with the LCD2-Classic, the LCD2, and the LCD X or else the Dan Clark Audio (formerly Mr Speakers) Aeon, or the HiFiMAN Ananda or Arya (although I don’t recommend HiFiMAN because of its notoriously poor build quality). Full disclosure: I have the LCD2-Classic and am conflicted and ambivalent about it: it’s a great rock headphone, and it’s good for electronic music, but it’s not an all-rounder; acoustic instruments, for instance, won’t sound great on it. I’ve had mine for 10 months now and still can’t decide if I’d like to keep it long-term.
Keep an eye out for Black Friday sales, especially if you’re in the US. I picked up my LCD2-C for $200 less than the retail price last year directly from Audeze.
Finally, you make an interesting point about the upgrade paths, or leaps, for over-ear headphones vs. IEMs. Just a couple of years ago, new over-ear releases in the $500-1000 range, with the Aeon, the LCD2-C, the Elex, among others, generated a lot of excitement precisely because, as you note, there was little else between the affordable and high ends of the market. I have to say I’d found that until very recently, the last year or so, the leap beyond $200-300 IEMs has been even more daunting. There were very few offerings until you got to the $1000+ range, with something like the Campfire Andromeda. Now, it seems, there are quite a few more mid-range options with the Thieaudio, the MEST Mini, the Moondrops, and others. It’s great to see this market open up!