Self host mastodon [1] and put it behind several CDN's [2]. Have multiple domains that are each created from unique registrar accounts on different registrars. Ensure your userbase bookmarks all the fallback domains. I have no idea which registrars are least likely to yank a domain offline as I have never hosted anything political. Keep the TTL for ALL your DNS records low so you can ditch a bad CDN or DNS provider. Your legal team should have a contract with each registrar and CDN that explains and accepts up front what type of content you are going to be hosting and what the registrar and CDN have to do in order to cancel your accounts and what is expected from your abuse team. I am not a lawyer and this is not legal advice.

[edit:] Oh right, ensure you have regular backups, encrypt those backups and distribute them to multiple servers on multiple ISP's. Or have something like postgres+citus in each location and do local backups at each hosting provider. Get the same legal agreements with your hosting providers.

[1] - https://github.com/tootsuite/mastodon

[2] - https://www.cdnoverview.com/#cdns

Thank you, this will now be searchable on google and @Parler #Parler will be able to see this and use it for advice.