IMHO installing a sustainable and efficient server is not a simple endeavour. This may not matter most in small scaled service groups and for home users.
“Too many cooks spoil the broth.” & Beware of “quick and dirty.”
First, keep it small and keep it simple. Consider if your Nextcloud installation actualy needs the ‘php-fm’ module as this is the NC home user forum, if I may recall. Trust the true experts in the background and there is good reason for an advice like "Note that most Apache users probably want the libapache2-mod-php7.3 package. " on Debian. However, Nginx is an excellent workhorse for big scales and in addition may help on systems with narrow resources.
Second, don’t cross your bridges before you come to them. There will be an official update to the php7.3-fpm and rhe nginx package available in due time as should be appropriate for your system flavour. There should be an applicable security advisory like e.g., a DSA available too, I presume.
Naturally, you are free to use your NC install for trials and to load any software as you deem necessary. Many if not most of the comments in this thread may guide you and provide true help, hopefully.
Hope this helps.