Adaptation is a form of growth…
You could, however, switch to per session standard milestones, and 1 per 3 players spotlight milestones per mission (assuming 2 to 3 sessions per mission), thus giving a significant boost to advancement
Note also that increasing time between level-ups is a standard trope, found in most experience systems. Either more XP per level as level climbs, or higher cost for an ability as the ability climbs, or higher cost per ability as total abilities climb. (I’ll spare y’all the list of which game is which method).