I replaced the bottom fan with an 80mm one and the same bottom cover you linked to. Bottom line, it is a bit of a hassle but the machine is much quieter now.

I used this fan:

https://www.amazon.com/80mm-10mm-High-Output-8010/dp/B01FBPQR7I

Be advised that while this fan is quiet, it doesn't come with a connector (just wires) so you have to attach your own JST connector.

For the cover, it's too big to print on the AD3 so you have to split it into pieces and fasten the pieces together (I used glue and printed some reinforcing strips). I didn't get a great fit after doing that so I also need to enlarge the screw holes in the cover. Plus the fan housing portion is bigger than it needs to be and I didn't love the recessed areas for nuts to attach the fan - measure your hardware before you print it. At the end of the day it works but I kinda wish I'd just rigged a cover from a piece of sheet metal.

Last thing is you need to raise the printer if you use that cover, so you'll need to print feet. I found some on Thingiverse and scaled them down to a good height to allow airflow to the fan. I just left the original fan in place.

Like I said, a bit of a hassle but no cooling issues at all and it really is much quieter now.

For the other fans I'd love a quieter fan on the extruder too but unless I learn of one that for sure is better I don't think it's worth it to replace.