You can make it permanent if you cast it 53 times, and by the time you hit level 15, 500gp for a week of downtime with comfort and security is occasionally worth it. Our druid/cleric regularly casts greater restoration rather than wait for me to prepare remove curse the next morning.
Yeah, fair enough. Casting it enough to make it permanent would be 26,500gp. Which isn't a tiny amount in a general sense, but by level 15 it shouldn't be a major amount of gold honestly. I'd be more concerned with finding that many diamonds frankly.
You don't even need to homebrew. Spell scrolls exist. It's not unreasonable to think a spell scroll of mighty fortress would be stored away for safekeeping somewhere for a long time before the PCs find it. 👍🏻
The spell mighty fortress is very specific about the size and shape of the castle it makes, but not about where walls are connected. Both are 120' square areas with four 20' turrets connected by 80' walls, but the second one you get more interior space and can access your turrets without leaving the outer walls.
Long story short: i like the idea, but the execution leaves a lot to be desired, and it's basically unusable at this point. It's very unbalanced, some buildings are clearly more useful than others, and having one bastion for each player is overkill. It also requires the campaign to stick to a very specific time schedule (a few weeks per level) that I honestly find very difficult to fit in any of the pre-existing modules, and certainly not any of the homebrew campaigns I've played with my friends.