After 4 years of mining on many video cards, fans have broken.
This is what most of my graphics cards look like now.
Some have 2 or 3 external 70-90mm fans
A video card with a Palit Dual cooling system had an anti-record, the fans worked for 18 months
I have these type of msi brand GPUs I would like to know what fan speed you've been running them for those years before the stop working, I'm presently running mine at 72% and I think that's safe
It depends on the quality of the fans. In winter, my fans mine at 60-70%, in summer it can be 80-100%.
I'm not even going to repair the cooling system when I sell graphics cards. The disadvantage I compensate the buyer with a lower price.
Major fan problems begin after three years
If you are going to replace a fan with a case fan then you should remove the dead fans out of the way first. Best is if you remove the entire shroud actually. However depending where the screws are you might void warranty that way. However a cheap case fan usually even 1 can do a better cooling job than the regular fans.
Look at the photo. The distance between video cards is 12-15 centimeters.
This does not greatly affect the cooling result.
It is enough to buy a fan with the maximum amperage 250 mA ( 0.25*12= 3W)