By all means stay with the MP 5,1 if you like tinkering, the expandability, and using only older apps.
I find it could be expensive and pointless to constantly tinker, add new components, and deal with kexts and hacks to keep the MP 5,1 updated and upgraded. It‘s a never ending battle to keep it from obsolescence. It is obsolete already. I enjoy the tinkering, but it’s not a hobby and I can‘t afford to continue throwing money and time at it.
Expandability on the 5,1 is way overhyped, because limitations come with obsolescence. Even if you stick the latest hardware in its aging body, it cant’ be expected to magically work as a modern computer. The built-in tech is 2 or 3 generations behind, e.g., PCIe is gen2, no Thunderbolt, no Bluetooth 5, etc. The 5,1 is stuck in time, which is perfectly fine if you only use old apps and don‘t need tech that meets the newest standards.
Take FCP classic, which is perfectly fine on the MP 5,1, but it does not run on macOS beyond Sierra. That means it’s limited to the last generation of graphic cards, like the RX 580 or a hacked GTX 1080. Sierra does not support the current generation of AMD Vega, Navi, or Nvidia Turing cards. (I have tried installing Retroactive, and it’s not yet ready for prime time for old FCP.)
FCPX is optimized for Intel CPUs found in newer Macs. My 2018 iMac — not to mention my 2019 MBP 16" — have no issues with timeline scrubbing or stuttering like the MP 5,1. My iMac is also faster at rendering, transcoding and grading. If I were to spend money for a new FCPX machine, I would not invest in the kaput classic Mac Pro, as much as I love it on my desk.