Do you plan to use some wifi router as AP only? Then you already have a switch there. Multi NIC bridging will take up some CPU power (not that much if you only use 1G, but not sure if it will hit SQM performance when all ports running 1G internally)
Non China branded device with multi port and having good performance probably PC Engines APU4? However I have no idea the availability in your country.
Then adding a switch should work for you, but if you have plan to separate all those room specific wifi then multi-NIC would make sense because you don't even need to deal with VLAN, just hook different wifi AP with different subnets.
Sadly, no. pcengines has become kind of a one-trick-pony, as they're still using the same old AMD jaguar cores they did a decade ago in the apu2. While good back then, they just barely manage to route 1 GBit/s with no margin left, sqm (at those speeds) is beyond the abilities of the hardware.
Unless you won't get close to 1 GBit/s or can get this hardware really cheap, I would skip it for ar least contemporary Atom or Intel core (i3) boards.
The problem, 35-75 EUR might already get you a used baytrail (Atom) based gateprotect, barracuda, trustwave or sophos system (sophos is easier to get, but also tends to be more expensive), which are much faster than those old jaguar cores...
That being said, you can get one of those Chinese N5105 4x2.5Gbe fanless PC's with NVMe for same kind of money. I think mine cost around 250$. SQM is mostly single core but the one you chose (3867U Celeron) will happily do SQM with a_piece_of_cake all day long @ 500Mbit with power to spare.
Re: moving configuration. It is not fully plug-and-play. I started with stock config, then edited config/network and config/firewall to look like old one. (Be aware that eth port names might be different and rename accordingly).
Also, you might want to resize the partition as stock OpenWRT will give you like 200MB.