I bought some half-inch silicone feet to separate mine from the shelf it sits on. The added airflow underneath seems to do just fine.

I recently installed OPNsense specifically because I had to buy a mini PC with 2.5 gig ports. There simply isn’t anything reasonable on the market for the prosumer above the 1 gig threshold. Running splendidly on a Beelink EQ12.

Also, OPNsense has things OpenWRT doesn’t offer (plugins, IPS, etc.)