I need to see exactly what the 2024 DMs Guide says about the bastion system. One of the main criticisms of the playtest bastion was its divorce from the normal economy that the Players Handbook describes. As far as I can tell, the official bastion will be moreso part of the normal economy − acquiring or building real estate, including an unlimited number of walls, kitchens, bedrooms, storage rooms. This normal stuff can be done at any level. Then the "special facilities" are the stuff that comes from leveling, not from economy. The special facilities are the part that can generate magic items and so on. So it would be possible for a character to have some kind of homebase even without special rules in the DMs Guide. Figure out the cost of constructing a wall, or buying a home or business.
Generally, the DM plays the setting, while the players play the heroes in the setting. The players decide how to build their characters, and these characters are "off limits to the DM". However, there are places where the authority overlaps, when something about the character has wider setting implications. For example, a background implies somewhere in the setting there exists a circumstance that makes it possible for the background to happen. In this context, both the player (for the character concept) and the DM (for the setting concept) need to agree.
The rules for the bastion seem to expand the definition of a "character" to include the home and business venture of the character. In the same way a player decides what kind of player character to create, the player also decides what kind of nonplayer characters populate the bastion. They might be family relatives, or hirelings, apparently even "recruited" monsters are possible. The player decides how to build and populate the bastion as part of the character concept.
When a player creates a character, the player often chooses aspects of the build that interest the player. So it is a good idea for the DM to riff of the character details for adventure hooks and resonant encounters that will be fun for the player. Likewise, when the players create bastions, the DM can see the kinds of things that the players find interesting, and to "bake" these interests into the wider setting, so as to entertain and keep the interests of the players.