dnd 5e – Is this fortress impervious to magic-based invasion and information leakage?

The defenses you describe seem redundant. The Weirdstone (which you’d have to homebrew, I guess) says that it prevents divination magic and planar travel. Then, the “uber-tight control over things and information leaving the facility” makes divination more difficult, the forbiddance prevents all planar travel, and the private sanctum prevents divination magic and planar travel. So you’ve prevented divination magic twice and planar magic three times.

If you’re willing to homebrew the weirdstone, it seems to me that you should just use the weirdstone. If you want redundancy, use two weirdstones.

(Why are we relying on weirdstones? Well, the other effects can be shut off by dispel magic. Someone standing outside the fort could cast dispel magic targeting your forbiddance effect and shut it down, and I believe this would work even if there was an antimagic field in the way.)

If I were trying to gain entry to the facility, I’d probably ambush and mind-control some of the guards when they left the facility for leave. I’d ask them for a detailed description of the contents of the facility. I’d use modify memory to make them forget they’d been mind-controlled — a careful reading of the spell seems to suggest that an antimagic field wouldn’t restore the memories. Once I controlled the guards at the facility gate, I could march the rest of my force right in.

You can counter part of this by using greater restoration on arriving guards, and probably you’d also want to use detect thoughts on them to check for moles. But I don’t know of a good way to prevent the thing where someone ambushes a guard who has left the facility and uses mind magic to learn everything they know. If your security policy is that nobody can ever leave the facility for any reason, people aren’t going to want to work there.