Tailscale is open source, it should be possible to set up your own server.
The hosted Tailscale product is meant for GSuite customers who want an peer-to-peer VPN with corporate SSO. Yes, you have to trust them - SSO login is inherently centralized. My company uses it, it works great.
I am not really sure you understand how it works. There is no hosted/not hosted versions of it. You must connect your "opensource" client/agent through their coordination servers hosted by them to host and publish the public key to the other devices in your network and you can not skip their service. So Tailscale is effectively as opensource as any commercial opensource VPN client. It's entirely useless when not used with their commercial service and users have zero control over the software unless when used with their servers. The "open source" thing is great from a marketing and business perspective because you basically benefit from the open source marketing and the community thing from the unsuspecting users and enthusiasts pros without giving away literally anything.
The hosted Tailscale product is meant for GSuite customers who want an peer-to-peer VPN with corporate SSO. Yes, you have to trust them - SSO login is inherently centralized. My company uses it, it works great.