1. I usually stick with distros that have large userbases. I've tried smaller and niche distros before, and inevitably they stop being maintained, or move in a direction I don't like. The larger distros like Ubuntu, Fedora, OpenSuse, have more resources (people, time, money) to spend on testing updates, and have reliable update schedules. When I was younger I didn't care about that kind of thing, but these days I use my PC almost exclusively for work 10 hours a day, 5 days a week, I need my PC to not break when I update it.

Another technique I use is to go to the vendor site for software I use and look at which Linux distros they officially support. Usually they will publish at least an Ubuntu package, sometimes a universal deb file that works on Ubuntu, Debian or Mint. Sometimes an RPM package for Fedora/CentOS too. This is getting less relevant these days with Appimage files and Flapak images that work the same across all distros.

It's natural to get bored or frustrated with one distro and want to try out others. Imagine if Microsoft made many different flavours of Windows that each look and operate differently, everyone who is bored and frustrated with default Windows would be trying them all out, comparing them, debating the pros and cons, communities would form around common favourites.

I have a small gaming PC that I use to test out other distros, I'm currently on Nobara, that I actually highly recommend for a gaming-focused distro.

2. This one is really hard to say. It depends on so many factors like what hardware you are running, what software you plan to run, how tech savvy you are, even your definition of what is an issue. Mint is very stable and easy to use, you may run into zero issues getting it installed, running VSCode, playing some Factorio. Or you might run into a small incompatibility between your GPU and the bundled kernel drivers and run into a whole world of hurt spending days tinkering on the command line with no usable graphics driver.

3. I believe Mint still comes with the Cinnamon Desktop, that is specifically designed to be familiar and easy for users transitioning from Windows. It's not super customisable, but I think it can do what you described. I'm not the best person to answer, I haven't used Mint or Cinnamon since 2012.

4. File extensions are optional in Linux for some kinds of files. Linux usually tries to identify a file type using a "Magic string", meaning it will read the first 8 to 16 bytes of the start of a file and will be able to tell with a great deal of accuracy what kind of file it is. Executables, drivers, shell scripts, and many others use this method and do not need a file extension. You can definitely still use extensions though. Eg, libre Office will still save documents with a doc extension (.odt). Often Linux will use a combination of both the magic string and the file extension to determine the file type. Eg, the magic string identifies it as an open office file, and the extension tells you it's a document kind of office file.

Your Linux photo editor will still save images with a .png or .jpeg extension, because these are the convention (and may be required if you will be opening those files on a different OS). Similarly, your project files created on Windows will still work fine on Linux (if the equivalent Linux app supports that file format).