The buildings from the regent guide are just examples; other buildings are possible, even buildings that help guilds.
When building something major like a cathedral I would:
- have at least +1 stability
- build it in a province with at least +2 prosperity
- ensure I have guild advantage working for me
- have a good court, say 6GBs
That's 6 points right there
- I would also make a nice action description for when I start the cathedral, to make sure I get the +2 bonus (that would carry over to subsequent free actions - no need for a new description each turn)
- I'd also ask the DM to allow me to hire a high-level expert to act as my Architect; if the DM is nice he'll allow me to hire him as a hireling that can only contribute to the building of the cathedral; granting a +2 bonus or some such.
That's 10 points total, and I can start taking 10.
Perhaps a bit difficult, but possible. Still, the DDC should perhaps be reduced to only 18 or the effects of the cathedral improved somewhat. Just note that this is a GRAND cathedral - not just an ordinary one - not every temple has one.