Startup Drive
If your Mac has a mechanical hard disk, one thing you could do to speed up startup and application launches would be to get an external USB 3.* SSD, clone your current system to it, and make it your startup disk.
Because your Mac has old-style Thunderbolt 1 ports that use Mini DisplayPort connectors and that run at "only" 10 Gbps, it would not be practical to connect a modern Thunderbolt 3 or Thunderbolt 4 SSD. But even a good USB 3 SSD would run rings around a mechanical hard drive,
RAM
If you have at least 8 GB of RAM now, a lack of RAM probably is not the cause of your slowness problem. Still, your Mac has user-expandable RAM and can take up to 32 GB. There are some cases (e.g., heavy Photoshop workloads, running virtual machines) where having more than 8 GB could make a significant difference.
Software
Microsoft and Adobe have a policy of supporting only the most recent three versions of macOS. Catalina is not one. Catalina is good enough to run the current versions of
- Firefox
- LibreOffice
- The GNU Image Manipulation Program (GIMP)
- The commercial Affinity V2 programs (Photo, Designer, Publisher)