Currently when I click on a RTF file in Nextcloud I am prompted to download the file. I see that I can right-click an RTF file and “Open in ONLYOFFICE”.
Is there a lighter weight rich text editor app for Nextcloud? If not, can I set ONLYOFFICE to open RTF files when I click on them?
Can I search for installable apps within my Nextcloud instance?
There is no alternative. Just the office suite apps available from the apps.nextcloud.com and your admin user panel on the top right under “apps”
I use the Collabora Online app and it works pretty well with rtf files.
See https://apps.nextcloud.com/apps/richdocuments
Edit: I found the answer to my below questions: Unintuitively, the search feature that searches a Nextcloud instance, and returns results in its own dialogue box, and does NOT return any results for uninstalled available apps, suddenly on the apps page interacts with that page itself and returns app results on that page. So there are two results areas, but only on the apps page. 
What is the easiest way to find the Collabora Online – Built-in CODE Server in the app section (or elsewhere?) of a Nextcloud instance?
What is the easiest way to find an app in general in the app section (or elsewhere?) of a Nextcloud instance?
Why should the global search show you results of uninstalled apps?
It searches in all resources of your NC.
Anyway, your question is a new use case and you can add an issue (feature request) at GitHub.
See my link above and search for it inside your app section. You need to add two apps.
Go to +apps as administrator and search for desired app.
I am looking for a search function within my instance. As my Edit above points out, I found it, unintuitively, on the global search of the apps page.
What you found is what we have right now. Agree that it can be improved. 