Hi Alex. I bought this card for the same reason as you, and was very pleased with it. It is very difficult to find a good quality card for this type of machine which is mainly for office use. This one is the best rated.

https://www.videocardbenchmark.net/gpu_list.p...

This card was mounted in an HP Compaq 8200 Elite Small Factor. The dimensions seem to me to be equivalent with the DELL Optiflex, to be confirmed by further research. In any case, it corresponded to my desires.

I have installed it in a Lenovo Thinkcenter M-Series. However, it is also necessary to think about the power supply which is often very weak on a desktop PC, I had 240W in TFX I changed it to 350W to be sure that it works.

Hello Zorglub59, thank you very much for your answer, indeed it must be compatible with SFF PCs. I'll test it and I tell you again, the Dell technical support advised me an AMD RX550 4Gb and when I bought it I saw that the dimensions were too big x)
See you soon :)