Ran EtreCheck on Mac mini (late 2014) : need help with next steps

It appears you are still using the original HD as startup, at 5400RPM it is very slow. You could improve this with a NVMe and adapter made for a Late 2014 Mac Mini. Or just get a SSD and replace the original HD. There are many instructions for this on youtube and other places, look them up with a search engine. I have the same make and model with both SSD and NVMe, it now boots in 20 seconds.

An NVMe is faster, They are both SSD, Solid State Drives. The other is a 2 1/2" drive the same physical size as the HD you have. It would fit without an adapter inside your machine.

A NVMe looks like the following Photo:

Both are faster than the old drive and have varying sizes up to two Terabytes. The NVMe has a socket on the bottom of the computer it plugs into with an adapter inside the cover. You could get an NVME of a smaller size for the speed and save money as a startup drive. Then use the other as extra storage and backup. The NVMe is more expensive.

I might add, if you are not wanting to tackle the work yourself see if there is an Apple Store near you, or, some Best Buy stores do this sort of thing. You could just add a NVMe and clone the HD to it making that as your startup drive. That is the simplest solution. Keeping the HD for storage of such.

I put SAMSUNG 860 EVO SATA SSD inside an old Mac mini 2009 running El Capitan when the old SATA 7200 rpm spinning HDD was getting way too slow. The form factor and the connector is identical so it was fairly easy DIY. I gave it to a relative for light use and it is WAY faster now.

I believe the slightly better (?) SAMSUNG 870 EVO SATA SSD has now replaced that TLC SSD. I'd avoid Samsung 870 QVO that uses QLC SSD because it might be slow with large writes and less reliable than TLC.

SATA limits the speed of those SSDs to about 530-550 MB/s while new M.2 NVMe is about 2500-3500 MB/s. But the old Macs might not support those speeds anyway? An SATA-M.2 adapter might also be incompatible in some weird way?

The storage of the drive has little to do with the speed of opening an App and boot up time. It is the read/write time of the drive and the CPU/GPU speed that matters. On your computer you cannot change the CPU/GPU. The following is an average time of drive speeds:

SATA HDD : 243 MB/sec

SATA SSD : 424 MB/sec

NVMe : 1726 MB/sec

The drive pictured above cost about $60.00 at Amazon. I know it is compatible with your computer, I am using one now. The adapter for it about $14.00, and if you do it yourself, the tools cost $11.00. It takes special screwdrivers to take the computer apart. Shop around, talk to other people. You are welcome.