When I do a Dell 9020 with Libreboot, any variant, I replace every fan with quieter Noctua fans which are premium quality designed in Austria - by comparison, the stock Dell fans are cheap, loud Chinese design.

Dell uses a non-standard connector, electrically compatible with regular PWM standard.

Adapters are available, but add greatly to cost and cable length, so I chop the old cable and solder it. Double-insulated; each wire insulated then all together, with heatshrink.

Here are photos:

I also do it for PSU fans, replacing the default one with a quieter Noctua fan.

The rest of the hardware in these machines is solid after replacing the RAM (I install new 32GB RAM, with 4x 8GB, and upgrade the SSD). I also de-lid the CPU and use quality liquid metal inside; and good MX-6 thermal paste outside it for the big cooler.

I use the best fans. Noctua are really really good. *Most* repair shops don't replace *power supply* fans. But I do!

Here are 2 photos, of different 9020 SFF PSUs:

@libreleah I've bought the 6 pin version of that connector on Aliexpress for a project. It was very cheap. Maybe it's easier for you and adds virtually nothing in terms of price to recrimp the fan connector with that one instead of cutting and soldering the wires.
Even better, maybe you can purchase only the metal terminals, recrimp the fan cable, and use the connector from the dell fan.
@libreleah The connector is called MX2.54 Dupont Connector.
This is the item on aliexpress, so you can see the reference:
https://a.aliexpress.com/_EHqGygJ