I don't have many problems with WiFi (I use an Asus RT-AX82U). But for my Synology and other mini PCs mentioned earlier, it's just better if they're on Ethernet. But despite my best efforts while they were building our house, the builders wouldn't put Ethernet cable in for me, so I must resort to retrofitting.
As I'm trying to avoid drilling holes in all my walls and ceilings, I first turned to Powerline adapters. But despite trying three different models from different suppliers, none of them worked reliably, especially across different floors of my house. This was the case in my previous house too, they just never work for me.
But like most new-build houses in the UK (our house is 8 years old), I have old-fashioned RF aerial sockets in each of the rooms, all of which go to an aerial booster in the loft. This is so I can receive over-the-air television, also known as Freeview in the UK.
After a bunch of hunting, I discovered this goCoax MoCA 2.5 Adapter (sold out as I write this). MoCA is known as "Multimedia Over Coax", which is a very American sounding protocol for sending ethernet over existing coax cabling. I bought two of these and installed one downstairs attached to my router and plugged into the aerial socket behind my TV, and the other one upstairs in my office. Along with a little Netgear Switch, it has allowed me to extend my network with minimal of effort.
I do have an aerial booster in the loft, so I did switch that off. I don't have any TV's plugged into the aerial sockets anyway, because my kids don't know what linear TV is. I'm not 100% sure if this is necessary, but I found some advice online that it was, and I did have some additional difficulty getting things to connect.
It's been 4 months since I installed this and it has been 100% reliable. No drilling required. I just wish there was an aerial socket in my garage and I could extend it further.