hardware recommendation for my mom

[Bamboszek]

I think you are really overthinking it.
Raspberry Pi is intended for DIY enthusiasts when you need things like GPIO, etc. It isn't really the best choice for the multimedia use.
Average mum would be much happier with something simple like Xiaomi Mi Box or Amazon Fire TV stick, with works out of the box without troubleshooting, tinkering or risk if some open source update would break the setup.

Same with the sound system. Relatively expensive AVR and speakers IMHO just would be be waste of space.
Placing speakers in proposed positions would make them sound awful, possibly even worse than TV speakers itself.

While I'm usually against it, just buy her HDMI soundbar which would fit under the TV and call it a day.
Simple, easy, looking fine and good enough for intended application.

 

[slacki]

It is the best choice for maximum flexibility i guess. I had a spare rpi (because i switched to an odroid n2 in my home cinema) and its now connected with wireguard to my system, so she has access now to all my videos that are on my NAS (and only to that ), works like a charm. Also, to have only 1 remote (that would be great), thats why i am looking right now for a usb-dvb-s2-adapter that can be controlled with tvheadend/iptvsimple in kodi, then also recording should be possible i assume (i am absolutely new to dvb-s2). I highly doubt that such things would be possible with e.g. amazon fire tv stick. But i never had one, so, not sure about that.
I got the AVR for about 320 EUR, thats imo perfectly fine. And the old speakers she has sound perfectly centered as if they are from the TV, even with that "bad" position. I was so surprised that i checked twice that the TV sound is really off. The small speakers are going to be replaced by two bookshelf speakers and maybe a subwoofer, but i will buy them used.
I first also thought i'll go with a simple stupid sound bar, but that would fix (maybe) only the sound a bit. Its multiple things that should be addressed like: 1) no advertisement (there was even annoying ads directly from the LG TV popping up all the time as overlay from the dvb-s2 signal, crazy), 2) better sound 3) "good user experience" aka access to some movies etc.

To address the problem with updates and a broken system: i disabled automatic updates, i will do that manually.

But yeah, maybe i am overthinking it. As long as it works, its fine