The amount of locked timegating on delve levels and previously prey levels feels pretty bad when many people went to farm them but were locked. This week with the delve event asking you to 5 delves, which is already a lot, it still won’t let you progress over level 4.
This doesn’t respect players time investment in the game and I’d personally recommend to simply make those system reward cosmetics only if the other rewards will block their progression when the content in itself is already available and being done.
Other type of rewards that aren’t cosmetic but also not tied to power could also be interesting. Like the delve teleport we had in TWW that could get us to a bountiful delve or having Valeera pick Mislaid Curiosity. We have something like that for the prey portal after you cleared one.