It's totally possible to have all the Kamihime if you're willing to spend for it. There are less of them than there are FKG and they are more common... yet there's people with all the FKG. It will break most of our banks but it is completely possible.

As for Gems I haven't seen it mentioned here yet but eventually DMM KH got an accessory system that is quite a gem sink... until we get it and awoken kamihime I would suggest banking as many gems as you can. Feel free to do a bit of the gem gacha now and then for fun but don't expect much, the new Rs that get released don't end up in it sadly.