Are you considering making the limited banners pullable with gems at a higher rate? Maybe 2-3 times a normal pull. Even higher might be doable, since you say you are not planning to put the heroes from those banners in the permanent pool.
Viewing post in Covenant of the Four Suns comments
Hey! Thanks for playing. You can now pull the limited banner with Gems if you're out of Crests, but it's a steep 500 Gems per pull (5x the standard cost). I went with 5x instead of 2–3x because I still want Solar Crests to be the real currency for limited stuff, but I didn't want you to be totally locked out if you're dry on Crests.
Basically: if you have Crests, it uses those. If you hit 0, the button flips to Gems automatically.
Thanks for the suggestion; those fixes will be published with the next major update! <3☀️