Etrecheck assistance for 2019 Retina 4k 2019 Sequoia 15

Please determine if the same problems occur in "Safe Mode": Start up your Mac in safe mode - Apple Support

Security updates disabled - Security updates are disabled. This computer is at risk of malware infection.

Apple security disabled - Apple security software is disabled. This computer is at risk of malware infection.

You might want to fix that.

Having had a look through the report, there isn't anything critical that stands out. Which is actually quite good. So, my best advice boils down to:

1. You have 8GB of RAM which is a bare minimum amount. The RAM usage info near the bottom of the report suggests your memory pressure is a bit high with swapping happening. There is a lot that gets loaded on startup and this would partially explain why your startups are slow.
2. You have a fusion drive (a virtual amalgamation of a SSD and HDD), but the SSD part is tiny at 28GB(?) - so almost everything is loading off the HDD except maybe the core OS and some heavily accessed files. That is slow and will stress your already maxed out RAM - explaining the report of being I/O bound.
3. There are a few old software components that could be removed (SoundFlower, Java plugins) and some that are probably also causing I/O stress (Google software, OneDrive).
4. Security updates should not be disabled.

The fixes you need are more RAM and at least replacing the SSD with something decently bigger (if not trying to replace the fusion drive outright). Which would both require service by an AASP.

But honestly - as you mentioned - it does boil down to "2019" and you might be better off spending any money on a newer machine.