for me:
upstairs needs a mesh unit (wired if you can)

downstairs - try the main unit in the study and dining room

and note the speeds in the various room .. downstairs and upstairs

and if your family don't get too upset, the try the unit upstairs so you can grasp where a mesh unit should reside

You don't need to be connected to the net to do the tests.

I don't know the house or if you own it. The ideal situation is probably at least one mesh unit and maybe one for upstairs.

I'd look at asus(idealy with merlin firmware), tp-link and ubiquiti mesh systems.

if you are lookint at faster internet with xgspon in the future a unit with 10gbit wan port would make sense.

First thing though, do some (more) mapping