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at your scale, it always feel safer to do a side by side to be honest - in place should work perfectly fine, but you lose the chance to maybe refresh the OS, and simply have a more rigorous and controlled migration. I just scoped one at a similar size to yours and honestly, side by side was just way less intrusive and risky. There are some tools to help replicate configurations and whatsuch to reduce the effort from a CVAD perspective

I tend to like to do in place upgrades where possible and introduce OS refreshes afterwards. Just did one for a customer of 3000 users where they moved from 7.15 CU3 to 1912 CU1 and everything went fine

Yeah definitely time and place for both, we have a few in places going on as well due to no need to change OS

There is no need to refresh the infrastructure OS in my case but yeah I have always been conservative and chose the side-by-side when possible. It is time consuming even if there is tool to replicate the configuration, I also used them in my previous migration. I was planning to do this on a Friday and have the week-end load (we have a few users logging in) validate the migration. In one hand I want to get over with this migration, on the other hand I don't want a P1 if I miss something during the week-end! Anyone did the upgrade in-place at my scale?

done inplace upgrade 7.6 -> 7.15 ->1912 for 10.000+ concurrent user platform (phased -component per component- of course); make sure to use latest CU before starting and never mix version for a component (ex no Storefront 7.6 together with 7.15 ... upgrade all SF)

Indeed, that's the way I would do it, thanks a lot for sharing your experience!