You're right that it seems a bit slow at first glance. 30 XP per event is not a lot.
However, the XP from events, hexes cleared, and unspent RP does add up over time. It just may take several in-game months.
The milestone bonuses are fairly small to start, but eventually level up to 500 XP which helps.
As the kingdom grows, more hexes are unlocked which means more XP per turn from them.
Additional buildings like Academy and Monument can boost XP gain as well.
Some GM rewards or special events may give big XP bumps to help advance the kingdom.
The intentionally slower pace makes each kingdom level feel more meaningful.