People have been looking for possible new material sinks.

One option is to make upgrades craftable. Instead of buying your upgrades straight from boost, you can craft them at an alchemy station. The energy cost of the upgrades would not have to change,* but the crafting would also cost some materials (preferably ones that are currently in low demand), and maybe some crowns (although that doesn't seem really necessary.)

Since upgrades expire, this introduces a recurring sink for materials for players. For the upgrades currently available from boost, you could introduce craftables with variable duration. Higher duration requires higher stat mats, and a better energy/duration ratio. It is important that there is an easy too access version for the 1st tier players, that requires only mats up to 2*.

An additional advantage, is that the upgrades appear less as an "Pay to win" option. To new players, the fact that currently there are these upgrades that obviously cannot be bought with mist energy alone, may give the impression that this is one of those F2P games, where one is expected to buy one's way to success. Even though nothing really changes energy wise by making the upgrades craftable, they feel more integrated into the rest of the game.

Besides the upgrades currently available from boost. You could also include a craftable mist tank. The tank would cost 100 energy to make plus say 1000cr and some really common mats such as a gel core. Since they cost 100 energy to make, you cannot use them as a way to convert materials into energy. Instead it works as a way of storing mist energy for later use, at an additional cost. This option may be interesting to casual players that, for example, only have time to play on the weekend, at which time they may want to play slightly longer than one mist tank allows.

*A slight decrease may be in order to compensate for the extra cost in mats and maybe crowns.