Solid plan, and the costs look right. Three things I'd tighten. First, one VPS plus daily backups is recoverable but not hard to shut down: if that host or domain goes, the community is offline until someone restores it. Get a second member's Pyramid running as a live write point early, not at 300+ members, and keep the community's relay list pointing at both. Second, Bunny.net is a normal company with terms of service, so it's the easiest takedown point in the stack; fine for media, but keep the Blossom originals on a box a member controls. Third, disappearing messages are a request, not a guarantee (NIP-40 expiry): any member's client or another relay can keep a copy, so tell members that rather than promise deletion. The invite tree with pruning is the strongest part of the design.