This right here. get something cheap, throw opnsense or pfsense on it and start learning. It will probably be incredibly frustrating at first but when it starts to click then it is really fun and rewarding.
I bought an old dell r210ii years ago and threw pfsense on it then swapped to opnsense and could not be happier. It is still in use today, a good 6 years later.
mine can push a gig around no problem.