I first started using Pound, as using nginx for SSL proxying only was a little excessive in my eyes. It's very fast, but at the time I had to use a Git version and not a repository version as Comodo certs which we used required SNI support. But it is worth checking out.
fpm is fine, FastCGI always seems to be problematic, and fpm is often employed as a fallback with hacky code when this is used anyway.
It's probably worth moving to Nextcloud. I remember setting up OwnCloud a couple of times and it was not easy, it's never been very well documented, possibly for 'support' reasons.
apache2 proxying is always a bit tricky, and it's not worth it. I like using Varnish for that, as the VCL is very easy to edit (C like):
- terminate at nginx
- backend (such as apache2)
I find this combination of nginx for SSL termination and apache2 work well. I got rid of Pound because I thought that we may move to nginx in the future, and it was better to do it all with one daemon, but that never arose. If it isn't broken...
It is -- because you'll spend more time writing VCL to set up cache exclusions than if you waited for pages to load