Hell yes! I have two Mac mini's and the 2012 Mini had the 5400rpm drive. I replaced it with an internal SSD. The boot times in Catalina were impressive. I went from around 1 minute and 28 secs to 40 secs boot time with an SSD. If you can get a SSD stick in that sucker you should get better results than I was getting.You can install a NVME in the 2014 Mini? I did not know that (figured everything was welded to the motherboard and the case itself is also welded shut). Would there be any appreciable performance increase?
But you won't pull the speeds I get on my 2018 Mini. I can boot Big Sur in about 7 seconds.