I don’t really see the point in hoarding those exp cards too much. I’m only at lv109 but trying to keep them made it harder for me to progress quests in new maps so I’ve basically come to the conclusion that you should just spend them if you feel the need to level. This might be different later in the game when you run low on card sources though.
I also got stuck at lvl75 around the Royal Mausoleum section of the Klaipeda chain. My level was too low to activate quests. I grinded a level and unlocked the next set of quests, but the exp from these quests didn’t get me to the level for the next quests. Then I realized that there are other maps closer to my level with quests I could complete. I started from the north east exit of Klaipeda and I jumped 10 levels in a few hours.
For now it seems like you just need to find the right map chain to follow to keep leveling while using cards as you need them.