The most stable MacOs/PT combo now?

[sixpointmusic]

I'm upgrading my macpro5,1 to 6,1 trashcan.
What do you guys think is the most stable MacOs/PT combo in this age.

I can basically have any combination, OS Yosemite~the latest and PT(regular one, not ultimate)12~the latest.

My current setup is this. I get many cpu spike/overload errors despite the 1024 buffer and a few crashes everyday. And offline bouncing is really slow. I do film score so lot's of tracks/plugins and VEPs (Vienna Ensemble Pro). Samples are read from pci 6g ssd.

Mac OS10.10.5 (on sata ssd)
Protools 12.5.0
Mbox Pro 3
Mac Pro5,1 Mid2010
2x2.66Ghz -6-Core Intel Xeon
Ram 64GB 1333Mhz DDR3
ATI Radeon HD 5770 1024 MB

My new Macpro 6,1 I'm ordering is this.(I can get any OS):
2.7GHz 12-Core
128GB Ram 1066MHz DDR3
Dual AMD FirePro D700 GPUs

I heard PT 2019.5 & Mojave 10.14.5 combo is very stable a few times.. has to be that exact combo?

Any advice can be helpful. Thank you!!