Most likely your current performance issues are due to the BX500 SSD. This is a low end budget economy model SSD that can be as slow as a hard drive during heavy use and it tends to have a high rate of failure. The MX500 SSD would be a much better choice. I agree seeing the DriveDx health report for the SSD would be useful to see if anything shows up since a manual inspection & interpretation of the health information is usually best since not everything will trigger a warning or failure. May as well do the same for the original Apple SSD as well.
The memory upgrade could definitely be an issue depending on the memory used. Macs are very picky about the memory they use so most forum contributors will only recommend using memory from Crucial or OWC. Even then, you should only use the exact part numbers recommended by using the tools on their respective websites to identify compatible memory. Buying memory based only on speed is a sure way to have issues since there are a lot more timings that are equally important for compatibility, but Apple doesn't post that information and the memory vendors don't always show the information on their product pages.
I see you had CleanMyMac installed recently. CMM has an awful reputation on these forums for causing all sorts of problems. You don't need to use any anti-virus apps, cleaning apps, or third party security software on a Mac. These types of apps interfere with the normal operation of macOS and usually causes more problems than they solve plus they impact system performance. Only if you suspect a malware or adware infection, then you can scan the system using MalwareBytes (no need to keep it installed or run its real time scanner).
I know the Wacom tablets & drivers have been causing compatibility issues with macOS 12.x Monterey especially with external drives. Some users have had to disconnect the Wacom tablet in order to access their external drives (usually they can reconnect the Wacom tablet once the drive has been mounted). macOS has experienced a lot of compatibility issues with new issues occurring with each new update (sometimes fixing an older issue in the process).
Try running Disk Utility First Aid on the Fusion Drive or on the hidden Container (I cannot tell for sure if you have a Fusion Drive setup with these two SSDs). Within Disk Utility you may need to click "View" and select "Show All Devices" so that the hidden Container appears on the left pane of Disk Utility. Even if First Aid says everything is "Ok", click "Show Details" and scroll back through the report to see if any unfixed errors are listed. If there are any unfixed errors listed, then you will need to run First Aid from Internet Recovery Mode, otherwise you will need to erase the drive (Fusion Drive or Container) and restore from a backup or clone.
FYI, the only useful diagnostic is one which reports an issue. A passing diagnostic does not necessarily mean the hardware is good. I would suggest running an extended memory test by booting from a Memtest86 USB stick.
