I was hoping there was maybe a completing "dating site template" already built in some technology that I could use as a base.
That might be a good place to start. The problem that a dating site solves is actually quite general. That is, link up people who don't know each other over some common goal. I think I once saw a "dating site" for manure, to link up livestock producers with excess manure they couldn't utilize (maybe not enough crop land) with crop producers who otherwise would purchase their crop nutrients as commercial fertilizer. Once matched, you get a win-win situation for both economically.
I think everyone is scrambling to adapt to the new computing world, where desktop is no longer king, or even relevant in many situations (like with your idea). Even a Web site needs to be mobile-optimized since that's often the way users first come to the site, not from their desktop browser.
But Web apps are not trivial, despite their apparent advantages in deployment and accessibility. However, once you've determined what information you need to log, you ought to be able to break the app into two parts, the server and the client. The server part you could probably write independent of how you eventually plan to implement the client part. In fact, you could have multiple clients - eg, a native mobile app too. You could start on the backend once you understand the problem well enough and postpone the client side until you've decided on what JS library or approach to use.
The UI of Web apps is also harder to do in the sense that because there really isn't any standard or best practices for how the UI looks and behaves (the way there is with, say, iOS or macOS or even Windows), a lot of the success of a Web app UI is really an aesthetic one - can you design a UI that resonates with your customers.
Also, don't forget about the expense of having an attorney write up your terms and conditions document and your privacy statement document. If you're operating in Europe or have European (EU) customers, you also have to comply with GDPR, which went into effect on Friday.