As an Idris owner (and someone who personally farmed over 20 million bayld outside of drops/purchases) I think you're exaggerating your HPB rate. Looking back at my 2+ months of farming bayld/HPB, Lair reives where I was soloing the reive from 100-0% were about a 3-4% drop rate for HPB and colonization reives were a bit better, somewhere between 9-17% depending on the area. You do occasionally get runs where you're hitting 3+ HPB in a row from reives but those were anomalies. And continuous farming of bayld/HPB is possible, but none of it is terribly efficient.
Legend assignments? Let's take a best-case scenario where you already have the imprimatur timer down to 5 hours and get max bayld per assignment (Dho/Woh Gates Peacekeeper's, 5000 xp/bayld for 1 imprimatur) you get ~33 assignments per week * 5000 bayld is 165000 - or 16 HPB. More realistically you're doing Courier's Drifts to Biv#4 for 4750/assignment and getting 156750 (15HPB)/week. Every little bit helps, sure, but a week of assignments gets you one tenth of one percent of the way to an Ergon.
There are a few places where you can bounce between Lair Reives if you can drop them fast enough (I preferred the lairs in Foret between Bivs #3 and #4) which gives you two lairs every five minutes. The deeper Foret lairs give you an average of 2500 bayld/reive as long as you're alone (2000 for the #3 reive, 3000 for the #4 reive) so that's 5000 every 5 minutes. That's 60000 bayld/hour plus whatever lucky HPB drops you get. That's decent, but I found it better when nobody was in the zone to do a full clear of the 10 Colonization Reives around Foret and then hit the Lairs while you wait for the Colos to repop. Each Colo is ~3000 by itself alongside a better HPB droprate, plus you get another two easy lairs by the FS that give 1000/1500. A full Foret run takes less than a half hour and yields ~37500 bayld + the 30000 from a half hour of lair bouncing.
If you just want Colo reives/want to make cash outside of the HPB you can do the Ra'Kaznar Colos since they have a 15 minute repop. You have a ~3-4 minute walk between the two reives but generally you can kill both before the first one repops. That reive also ends up giving ~6000 bayld and a much higher HPB drop rate (plus those high-value Ra'ka heads) so a lower bayld/hr rate of 48000 might be something you're OK with. This also has the benefit of being a long enough reive that having other people there generally doesn't cut into your personal bayld count all that much.
Overall I think HPB/bayld conversions in general are in an odd sort of spot. HPB doesn't have much of a use for at least 95% of the current playerbase (and I think I may be underestimating it considering the current # of completed Ergon weapons across servers) so the price should be low. On the flip side, dedicated bayld farming - unlike other types of currency farming like Dynamis and Salvage - provides a much lower raw number, no real benefit for non-WKR outside of the bayld (capes are ok, but most people likely already have the ones they want) and actively penalizes having more than a few people doing it at the same time due to repop timers. Due to that, plus the apathy of the general server population to actually farm it, that keeps prices high, especially for those who DO farm a lot of it and understand that bulk sales will be valued higher since buyers hate picking through bazaars for random 1-2 HPB sales.
As Ophannus said, I think a push to add more incentive to doing reives could help. Maybe the rush for Alluvion Skirmish parts will help with that for a time. We'll see next week!