Are there any good fixes for late game performance or is all hope lost?

There's nothing you can do from a hardware perspective; you have a very good CPU for this (though it is quite old--consider upgrading soon!), which is where the performance bottleneck comes from; the devs as you know need to spend a lot of time optimizing the CPU calculations.

From a game-play perspective, your best bet is to only use certain play styles:

• don't use slaves or visit the slave market.
• optimize for pop unemployment rather than unfilled jobs.

Each tick, the game checks for unfilled jobs and tries to assign people or migrate pops to planets that need jobs. You will get lots of unrest, crime, etc. optimizing for pop unemployment instead, but it makes the game playable, at least.

For reference, I am able to play through year 2700+ on a large galaxy at a speed of about 1.5 game-days per second by playing with slavery disallowed and with all my planets having more pops than there are jobs available. My specs are:

• Ryzen 5 3600X - stock
• 32GB of DDR4 2133 RAM - stock
• Samsung NVMe M.2 SSD
• GeForce GTX 1060 6GB
• And I play on borderless windowed fullscreen at 1920x1080 resolution with most video settings set to high or max. My population at this time is around 15,000 and I have a few hundred planets.