GT-AC2900 running at 82°C on Asuswrt-Merlin 386.1 - too hot?

[akamaka]

Should I contrive some sort of mechanical swooshing device to make sure the fluid circulates adequately?

You could but the it will all depends how big the tank is.
If you want you can use a USB water pump to create some liquid movement (powered from the router USB).
https://www.amazon.ca/dp/B008OCZUK6/?tag=smallncom-20

 

[akamaka]

Should I contrive some sort of mechanical swooshing device to make sure the fluid circulates adequately?

BTW; I am using a USB powered Thermaltake laptop cooling pad for my RT-AC5300. Has speed control and you can tune it, for noise level/efficient cooling.
I bought it about 6 years ago when I got the router and the dust is not an issue, although if you are concerned you can also use a thin filter in between.
See below the current temps with 23°C ambient. In the summer the router temps newer exceeded 65°C

Current Temperatures: 2.4 GHz-45°C; 5 GHz-49°C; CPU-60 °C