Would the MacBook Pro or rpi4 with a second Ethernet nic running a firewall before the routers also fix the issue of not getting security updates?

No. For most routers, this provides no additional protection to the router. Your router should not be accepting connections from the WAN side that would be blocked by the firewall, but consumer routers almost always initiate connections to the WAN side, indistinguishable from normal client traffic to your firewall, and accept connections from the LAN side, invisible to your firewall. If the firewall blocks all incoming requests, it would create problems for UPNP, effectively giving you CGNAT, even if the firewall does not perform address translation.

At least for some laptops, you cannot just remove the battery. If the battery is removed, the performance may be throttled. This is true of very old MacBooks.

In the US, most IPSs have remote access to your modem as well, even if you purchased it yourself from a store unaffiliated with your ISP.

Enabling SSH password authentication is unnecessary and not a good idea, especially if your temporary passwords are simple. I haven’t used Hetzner but there is probably a way to upload a file or to paste into the console, or else if you fix your keyboard you could at least type a URL to download the public key from the internet. You may want to look into cloud-init instead of manually installing and configuring your VMs.

LUKS may not make your server meaningfully more secure. Anyone who can snapshot your server while it’s running or modify your unencrypted kernel or initrd files before you next unlock the server will be able to access your files.

curl bash is not as bad as people think. Nobody downloads and reverse engineers binary packages off of these websites before running them with the same permissions.

If you’re running insecure services, you can restrict them to be accessible by vpn. I have a mix of internet accessible and vpn accessible services using the tailscale nginx plugin.

If you want to send all your traffic over a vpn, you will either need to route all your traffic through your own vpn or use some sort of multiplexed vpn. tailscale can do this with mullvad, but it’s not yet possible with headscale.

Helm is what is used for real world software deployments. It has its problems but it’s better than Docker Compose.