Hey Rik and Tim!
We think those errors should be the last! Yesterday our dev team shipped the final part of their work that should fully resolve your issue. I think the fix was shipped a couple hours after your last report.
While you are among the first to benefit from the feature-flagged fix, it will become our normal production path in coming weeks - we rushed to get it ready for you to confirm the behavior since your site was particularly triggering some conditions that led to problems, but everyone will be on the new system soon, so the fix won’t be lost nor you stranded out on an abandoned codepath 
I do not expect that you’d encounter any performance changes, either in site service nor in deploying, nor in the speed that your deploys go live.
Re: 502 errors, those are usually something around how we connect to proxied resources or functions and would not be related to the 500’s. Please send us the x-nf-request-id (cf [Support Guide] Netlify Support asked for the 'x-nf-request-id' header? What is it and how do I find it?) for a 502 you observe and we’ll be happy to investigate further.