The tips are maybe one or two milimeters away from the sidepanel. So it does fit, but just barely. Playing D4 results in temps of 70-80°C, which is absolutely acceptable to me.
The only issue I ran into was enabling DOCP on the RAM. I did not inform myself about RAM speeds and AMD processors beforehand and thus needed to CMOS reset after the system stopped booting 🙃
While opnSense is a fine solution (use it myself), I am not sure if your current router is actually the problem.
I suspect that the upload speed may be the culprit. Even when you are just downloading stuff there is data going bi-directionally. Even if it's just TCP ACK packets. If your upload is fully saturated, download speed will suffer as well or even cut out entirely. You may also consider that Windows / Steam / Epic Games etc. like to download updates in the background, which might also fill up the capacity of your bandwith.
Does your router provide some kind of statistics for current bandwith use towards the internet? What does it show when those halts occur?
As for your planned homelab. On a birds eye level I would recommend using a small cheap computer with two network interfaces and opnSense as a router / firewall and Ubiquiti Unifi Access Points with the unifi controller software hosted on your media server. I can go into some more depth if you wish, but the official documentation for opnSense is pretty good.