You only need one inbound machine as your bastion. Then hop from there to the rest using local address. Once you set up the proxy config in ssh it’s completely transparent.
Right yes but I (for various reasons) end up using a lot of different client systems and I don't want to have to configure all of them to transparently jumphost or use different port numbers and why are people spending so much time trying to tell me that I should make my life complicated in a different way to the one I've chosen?
It's weird how much pushback you're getting for a few simple firewall rules, but I guess it's just another bikeshed. Basically all of the options for doing this are simple if you already know them, and have some annoying complexity otherwise. So everyone has a favorite.
I've got a similar setup to what you've done here, with the policy routing and wireguard tunnels being part of a larger scheme that lets me granularly choose which Internet horizon each particular host sees. So I can have a browsing VM that goes out a rotating VPS IP, torrent traffic out a commercial VPN, Internet of Trash out a static VPS IP (why not separate from my infrastructure IP), visitors' devices going out a different rotating VPS IP (avoid associating with me), Windows VMs that can only access the local network (they have personal data), etc.
I'm currently hosting email/etc on a VPS, but the plan is to bring those services back on-prem using VPS IPs with DNAT just like you're doing. Any day now...
