This is only supposed to be temporary while you set up the FDE, your IP is unlikely to change in the 30 minutes or so it takes to go through these steps.

In any other scenario I’d close port 22 and use a mesh VPN with SSH capabilities, like Netbird or Tailscale. I’d only open it up again to access Dropbear during reboots, and I’d use IP filtering for that as well.

As for DHCP, I guess it depends on the ISP. I don’t have a static IP but mine doesn’t change as far as I can tell unless my router is disconnected for a longer while.