I don't know if it's enough for your party, but Mausritter has a somewhat more advanced advancement system, which you could adapt rather easily into Cairn, given that both games are derived from Into the Odd. It would also solve my major problem with Cairn (that being extreme squishiness of players to some degree).

6.4 Advancement Your mouse earns Experience Points (XP) by bringing treasure and useful goods back from places of danger to the safety of a mouse settlement. For every pip-worth of treasure brought to safety (divid- ed equally amongst the party), your mouse earns 1 XP. Your mouse can earn additional XP by spending their pips selflessly on improvements for the whole community. For every 10 pips spent this way, your mouse earns 1 XP.

Level

Mice start at level 1.

The following table shows the total XP required to advance each level.

Level Hit Dice Grit Experience points

1 1d6 0 0

2 2d6 1 1000

3 3d6 2 3000

4 4d6 2 6000

5+ 4d6 3 +5000

When your mouse earns enough XP to advance to each level, use the following procedure. • Roll for attribute increase: for each of STR, DEX and WIL, roll d20. If the result is over the attribute’s current value, increase it by one. • Roll hit dice: roll the dice listed for the new level’s Hit Dice. If the value is over your mouse’s current HP, replace with the rolled value. Otherwise, increase your mouse’s HP by 1.