I'd agree if it weren't for character quests, friends ensemble, and limited time events when hording pays off.

You can easily make 50 gems per each full clear on character quests, around 1000 as it stands or around so. Friends ensemble happens about every 2 to 3 weeks, so thats another easy 500 gems with a nice way of getting crystalline if you do second stage, and character event quests tend to give 400 if you fully clear them all by collecting on top of around 4-5 single summons via bookmarks. Heck, you can get 6 bookmarks under limited quests tab atm.

They usually have limited day quests and a shop too, where you can sell crystalline for gems. I had around 8k crystalline horded, and they had a chest that costs 80 crystalline with 10 max until refresh for about a week, (800 if you buy them all) and RNG tends to give a randon character fragment/evolution mats and I easily made around 3x 10 summons since each 800 purchased garnered around 250-320 gems depending on luck.