I agree on the outdated UI, but at least it gives you a ton of options that official apps would never allow you, or babysit you too much to offer.

I’ve literally used Calibre 2-3 times, and while the UI seemed daunting at first, it was logical and did did its job quite fast when figured out. And I usually don’t need to use it afterwards for another long period.

Good luck doing that to my 2012 5th gen Kindle that’s been off the update schedule for an eternity.

Plenty of 4k with HDR on Real Debrid. Or even better quality and bitrate ripped from BRs, in the open waters.