HoR is the best use of BGs best for progression. It really depends on your HoR line-up though desu. The onyx cost is the worst part of the deal; not just the 40k upfront, but the loss of 40 4*s as well which is 96k Onyx (without considering random extraction bonuses).
Naturally you can also do minimum pulls instead of maximum pulls and hope RNG softens the onyx blow. But it's generally best to wait until a double mileage event or HoR Mission Pass before doing that. If you do then remember that you need to work in at least 1 pull that's 12.5% since doing strictly min pulls isn't quite enough to max out the meter.
As the others said costumes and Scarlet Collection are also options. SC is less attractive for progression since you can only control for element and not six specific Childs, but if there's someone you don't already own and really want to try and pull for then it's fine. Nothing else for sale in the SC is worth it IMO.
DO NOT roll on BG banners. They are all trash. Except if you wanted to roll a specific element and there happens to be an elemental banner up. The difference between that one and the SC ones is that SC can only be rolled once a day. Someone with your ludicrous amount of BG could actually benefit from a banner with no waiting period.
There actually is another sort of BG banner that's good, but I haven't seen one in a LOOOOOOONG time. The type with four Childs divided evenly on them. Those can be good if there's at least one you want. However I haven't seen one of these banners since Line Games was in control, IIRC.