I don't know if you have me on ignore or something, but you don't seem to respond to my posts. The DMG already has rules for building a stronghold at any level (just takes money). The difference is you don't get the bastion benefits. The bastion rules do not replace the rules in the DMG, they augment them.
In
post #54 of this thread I provided some of the tables from the DMG. There is more information in the DMG too (and PHB).
EDIT: FYI Xanthar's doesn't really add anything to the guidelines in the DMG and PHB for building and running an establishment. It covers other downtime activities though.