The end of Cryptkeepers is intended to feel somewhat unsatisfying; that’s why the achievement for completing the game is called The Lesser Evil. You can’t improve your main stats during the game, and you can only improve your lore stats in chapter 2 (if you sit with the adventurers and talk about a subject you don’t know well) and possibly in chapter 4 (where it’s possible to boost your knowledge of the Flowering Court if you visit the sanctuary when you’ve already found out all the basic info about the Ever-Living).
Aside from upgrading lore stats, the main benefits you can get are (i) the Heart (which means immediate negative consequences for Hallowford, and unclear long-term potential benefits for you), (ii) increased gold, (iii) increased renown, and (iv) good relationships with characters/groups who might be able to help you (or at least avoid hurting you) in the future. In general, at least one out of Lucan (and the Cryptkeepers), Sir Michael (and likely the townsfolk), and Selim will end the game unhappy with you.
Instead of collapsing the tunnel, which makes the adventurers who survive upset with you (and means there are fewer adventurers to fight the Ever-Living), standing and fighting can boost your renown, lets you discover that the Ever-Living are weaker the higher up they go, and boosts your relationship with both Mundy and Selim. There’s no real disadvantage to standing and fighting if you avoided getting injured in the prologue (which is only possible for an imported character).
The two playthroughs I have that end the game in the best position, I think, are: Roland, who gets the reward from Lucan (tons of cash, decent renown) and ends the game on good terms with everyone except Selim, with whom he’s on decent terms (I put a lot of effort into boosting his relationship with Selim, so that they’d still get along, even after a hefty penalty near the end of the game); and Artemisia, my high subterfuge MC, who ends the game with the Heart but without everyone thinking of her as a thief.
There aren’t many opportunities to significantly boost your wealth during this game, aside from the final Cryptkeepers’ reward, but you can get some gold for reporting to the Cryptkeepers who stole something of theirs, some from stealing from the town watch or the Cryptkeepers while preparing for the final trip into the crypts (if you have the right lore skills, you recognize more valuable items to steal), and some while escaping if you stole the Heart.
Going solo is when Sir Michael lets you go or you escape from him, and then when Lucan presents his plan you walk out with Selim, but you decide to not join Sir Michael. You might have Mundy, Selim, and/or some other adventurers accompany you, or you might be completely alone. If you escaped from Sir Michael and you go solo without Selim, it’s possible to steal from both the town watch and the Cryptkeepers.