You already got some good answers. For me, the most inconvenient part of the transition was that I had to replace one device (a FritzBox 7590) with four devices: DrayTek DSL Modem, Protectli Hardware Firewall [1] with OPNsense, a 5-Port Netgear Switch, and a BrosTrend Wifi Plug (altogether, 21 Watts instead of only 10).
Regarding the Hardware Part: Instead of the Protectli, I would recommend looking into the Pc engines APU4d4 (or similar) [2], I had very good experiences with the predecessors of this board.
The OPNsense configuration is really not that difficult, it is possible to start slowly with the defaults and extend. I already had experiences from running pfSense for 5 years on another site.
Regarding the Hardware Part: Instead of the Protectli, I would recommend looking into the Pc engines APU4d4 (or similar) [2], I had very good experiences with the predecessors of this board.
The OPNsense configuration is really not that difficult, it is possible to start slowly with the defaults and extend. I already had experiences from running pfSense for 5 years on another site.
[1]: protectli.com
[2]: e.g. dynfi.com/en/appliances/dynfi-apu4d4/