If you want to go the raspberry pi approach you have to do some things by yourself in terms of configuring a network (but it is not that difficult).
You would need:
- raspberry pi 4 with proper case (maybe active cooling) and psu (with at least 2 GB RAM; would recommend 4 GB variant) ~75 EUR
- an usb-to-lan adapter (you could work without this, but network setup is more complex then) ~15 EUR
- a (web-)managed switch (with at least 5 ports; better 8) ~20-30 EUR
All in all you would end up ~120 EUR.
https://www.zahradnik.io/raspberry-pi-as-a-home-router/
If you have a Gigabit connection and want to max it out your raspberrypi is working at 100% cpu usage to reach this. If you enable sqm/bufferbloat to gain better pings then you cannot max it out anymore and you have to go for a real cpu like @slh said already. Especially if you enable VPN on top. VPN is a really huge impact here!
Think about your needs. Are your 15-20 devices demanding high bandwith up to that what your line is giving you at same time you are gaming and devices using VPN? Then go for that what @slh has written.