I’m in the same situation with 2008 Mac Pro and holding since I don’t forsee paying maserati prices for a new one.
I tried the C10 trial and found it unusable. Apart from the horrid transport bar, which is a feature, the mixconsole racks disappeared for no reason. There was some other buggy things I don’t remember since it was a while ago.
Be aware that if you do the trial, you will have to hunt down and manually remove some vstsounds that also expire when the trial ends since there is no uninstaller.
My advice is don’t bother. There seems to be changes that make new Steinberg products unusable on El Cap if they install at all.
One option I’m also considering is the Sierra patcher that can be found for free on google. It claims to only have a problem with the trackpad. There are also patchers by the same person that go up to Mohave, but the higher you go, the more issues.
If you go that route, please post how it works for you. I will do the same, though I think it will be a little while before I try it.
Right now, all I can say is damn apple for leaving no affordable middle ground on custom systems.