You can use whatever port if you’re proxying it to 8448 (assuming the container host is listening on that port which it would be by default).
If you use matrix.example.com and proxy port 443 to 8448 then it should work as your user would be @user:matrix.example.com.
You would need to portforward from outside your network if you want to access it and have it talk to other servers as they all need to communicate with your server.
Personally I wanted matrix to show my user as @user:example.com at my actual domain not as a matrix subdomain, so I had to setup the .well-known/server type http path to tell it my server is actually at matrix.example.com.
Depending on how dirty it is, I would assume at the very least cleaning the car removes things that can harm the paint (and result in corrosion later down the track) like bird poop, road grime and brake dust to name a few things.
Even just washing very infrequently, maybe a couple times a year could probably save the paint on your car and prevent premature corrosion of a vehicle!
Plus some people just like their car to be shiny I guess!