.NET+HTMX can be a decent experience
Since this is a simple app, I took the opportunity to find a nice way to use HTMX with .NET.
I sent my HTMX requests to a proper .NET API controller that then renders a hierarchy of ViewComponents which act as reusable website components. The required data is mapped at render time by the caller.
If you are interested in this setup, Source code is available here.
Credits also to Jetbrains for their tutorial series: HTMX for ASP.NET Core Developers, a good starting point for anyone interested. Their rendering setup differs from mine.
For the real-time autocomplete, I turned away from HTMX, because while server round trips for the main input boxes of an app are feasible, the resulting experience would not be good.
I wanted people to be able to play this game on the train etc. without any kind of lag. So I wrote some vanilla Javascript that fetches the country list and displays the autocomplete box.
Additional thoughts
HTMX should be fine in medium-sized because you can use your normal backend abstractions to keep the code clean. Just make functions that return HTML strings.
It is a bit unusual, and some people might hate it, but I did this successfully in a different project and it worked out fine!
What you do need is some sort of click-through-everything, end-to-end testing, because nothing is compiled.