Maybe LosslessCut: https://github.com/mifi/lossless-cut ? I never used it, but looks promising. HandBrake: https://github.com/HandBrake/HandBrake is also a great GUI tool to convert videos, with some basic editing (cutting and such).

You could also always learn Ffmpeg from commandline and with scripts. Obviously this has a huge learning curve, but once you learn it, you are then free from any GUI or other tool in the future. At least for simple tasks this could be useful when automating. It's on my todo list...

I use MKVtoolnix for reboxing files (naturally it outputs MKV). I would assume ffmpeg can do the slicing that you want but it's just a library with a CLI. So if you search for ffmpeg GUIs you can probably find one to your liking.

Is there any reason you need to switch other than the age of the last releases? Avidemux had its last release in 2022 and its last commit a week ago. For a piece of software with a fairly stable and unchanging set of features, that seems pretty reasonable to me. Avidemux still works fine on Linux and is still packaged by all of the large Linux distros, so I don't see the problem. VirtualDub's definitely dead, though.