Welcome!
Several things. First, you are trying to run Catalina on 4GB RAM. Fortunately, the computer was not starved for RAM at the time of the test, but could be once you have a lot of things open. My som is runnign Catalina on the same Mac with 8GB RAM and it is working quite well.
This is a cause of "slow."
Top Processes Snapshot by CPU:
Process (count) CPU (Source - Location)
Other processes 81.07 % (?) 👈🏻
EtreCheck 8.21 % (App Store)
trustd 5.06 % (Apple)
CoreServicesUIAgent 0.23 % (Apple)
accountsd 0.16 % (Apple)
To see what that is, you'll need to grant EtreCheck Full Disk Access. Instructisofor doing that are in Etrecheck's Help menu under the "more Help" category:

However, your big hitter is what plagues most Macs of that era with mechanical hard drives:
Performance:
System Load: 0.98 (1 min ago) 1.10 (5 min ago) 1.33 (15 min ago)
Nominal I/O speed: 0.78 MB/s
File system: 69.50 seconds
Write speed: 77 MB/s
Read speed: 81 MB/s
That is top performance for that drive model but the drive, like the MacBook Pro 2012 13-inch itself, is entry-level. It is a slow 5400rpm SATA 3GB/sec drive attached to a fast 6GB/sec drive bus. On that model a 6GB/sec 500GB solid date drive kit will cost under US$100. I put such a kit in my MBP 2012 and these are its drive scores:
Performance:
System Load: 1.61 (1 min ago) 1.60 (5 min ago) 2.94 (15 min ago)
Nominal I/O speed: 0.15 MB/s
File system: 28.20 seconds
Write speed: 487 MB/s
Read speed: 482 MB/s
It feel like a new computer. This is the kit I used:
https://eshop.macsales.com/item/OWC/Y3SSD6E500/
As you can see, there are other capacity options. As you have recently replaced the drive cable, that is all you need. I highly recommend the kit because it has a transfer enclosure that makes cloning the old drive to the SSD very easy.
Based on many posts here, people using OWC and Crucial drives have the best success.