> why would I want to buy custom hardware when whatever off-the-shelf 2013ish netgear/linksys router I have can do port forwarding and max out my connection?

for me it was because

- it could not max out my connection: it was maxing out a 70Mb/s on a 100Mb/s connection

- those off the shelf routers do not support vlans, and I like so segment my home lan in stuff I trust, and stuff I don't trust.