Hello,

Welcome to the HandBrake repository.

I usually add a lot of video files (from different folders and adding only the top-level folder doesn't make sense as I don't want to add all the videos in it) to Handbrake and I always use the same preset 99% of the time.
Would it be possible to create an option so that videos that have been opened or loaded into the program using drag & drop are automatically added to the queue?

Use the Recursive Scan mode. This can be enabled in preferences -> Advanced -> "Recursively scan directories"

If that's enabled, once you've scanned the files, choose the preset and then us the "Add All" option on the queue button menu.

I use the program to convert my old video files to the newer more effective x265 codec, mainly to save disk space.
How about an option that after finishing the conversion, checks if the new file is smaller and if so, deletes the old one.

Removal of files isn't something we plan to do. We cannot guarantee a successful encode and can't always tell when an encode is a failure. As such, deleting files comes with risk.

If you really want to do this, you could use the "When Done" feature and write a powershell script to perform checks and cleanup.