If you do want to go the web route, I’d highly recommend avoiding SPAs and going with https://htmx.org/ instead. Much simpler, less code, entirely driven by your backend, while still giving you the ability to make nice interactive applications.

As a bonus, since you presumably have been working with Python anyway, the author of htm has a whole book online walking you through building an app using htmx and Flask, a web framework for Python: https://hypermedia.systems/book/contents/

JS is optional, it’s unpleasant, as well as use a bunch of fat frameworks. But now there is HTMX, which makes any GUI without JS, pure HTML combined with some Bootstrap 5 is enough. I write in Go, it has a great built-in template, and it’s enough for any level of GUI

be as portable as possible

This is important to me, which narrows down my options quite a bit.

Electron is portable across desktop OS, but unacceptably bloated (I don’t want my users to have to deal with that) and buggy (I don’t want to deal with that).

wxWidgets and various similar wrapper libraries exist, but on Linux most of them wrap Gtk, which in recent years has become very opinionated in UI directions that I find intolerable.

A few new cross-platform GUI toolkits have been appearing recently, but I’ve found all of them suffer from poor text handling, anemic widget sets, or very out-of-place look and feel (especially keyboard navigation) relative to native applications.

That leaves Qt as my only reasonable choice, at least for now. This is mostly okay, as it does a wonderful job all around. My main complaint is that using the full power of its widgets and libraries means I’m restricted to a handful of languages: C++, Python, and maybe one or two minor ones like D. Its declarative API (Qt Quick) seems to be getting more language bindings, though, so simpler apps might be possible in other languages.

Note that the landscape is different for mobile apps. I don’t have a recommendation for those.

Time, experience and a lot of mistakes. Everyone who has been programming/scripting has made their fair share of mistakes along their journey.

Sometimes you just have to pick one, start it and see how it goes.

As a C# programmer, I like to make desktop GUIs with WPF or Avalonia. Web is also possible with asp.net, but I don’t like it as much.

Making a web app is a mistake 9 times out of 10, particularly when dealing with larger datasets. Because you’re in physics, you probably want to skills you’re learning to be transferable into physics and data science in general.

I recommend starting with python (if you know it already, awesome), then checking out pyqtgraph – there’s a bunch of demo apps that come with the package and you can use those as launch points. This will be your gateway into pyqt/pyside and legit desktop application development. Later, if you learn C++, you can transition into Qt (and still use all the power of the toolkit and the skills are transferable), or into raw C++ which is amazing for numerical computing.

You may be interested in MatPlotLib for Python and maybe something like mpld3 to get it to the browser. I’ve been trying a few frameworks out recently, so I’ve seen a few Python options that use the browser as their main window. NiceGUI has a browser front end and graphing that may be able to do what you want as well, but there are others you can find. You might look for tutorials on YouTube if you’re having trouble deciding, to see how difficult it is to learn and work with. If you know any c++, there’s a long but descriptive list of frameworks available on GitHub, sorted into categories, that could help narrow those options down. It probably depends a lot on what you already know, but there are more options than JavaScript. You have a lot of good keywords to narrow down a search. <Your language> GUI or graphing framework browser frontend open source (no license)

I am on my way to bed so I can only give a brief and simplistic overview:

In principle a basic web application runs in the browser, so it is executed locally on your device. The scripting language the browser understands is JavaScript. With JS and JS libraries you can do pretty much anything you would expect and probably more. In general, you use JS to do whatever you need to do to your data and then use it to manipulate the HTML document in order to display your data. You can also have a Backend, running somewhere else (on a server) and using whatever language you like as long as it can then communicate with you frontend JS application using the appropriate web protocols. You dont necessarily need a Backend, a lot of stuff can be done using JS that is executed on your device. This depends on the use case (e.g. for fetching data from a database on your server you could write a Java/Python/whatever app that runs on your server, fetches the data and sends it to the client where it is received, processed and displayed via a JS app).

Now to write a JS app, most people nowadays use some sort of framework (react, vue, nextjs etc). This is quite a different process than writing a vanilla JS app, might take some getting used to depending on where you are coming from but is worth it in the end. If you want an easy start I would recommend vue3+pinia store.

There are also GUI libraries as well as styling/CSS libraries which can be combined with this approach (or not). Take a look at NaiveUI and Tailwind as examples.

Thats all I can offer, best of luck and good night!