I come here after Userscript Youtube+ written by ParticleCore, mostly because when Youtube adding new stylings and ParticleCore don't have time to work on rewriting his script under it. It have features what this addon still not have since first day youtube released styles for all. I hope you can look at his addon features: https://github.com/ParticleCore/Particle/wiki/Features and as code is public you can get an examples.

What features I really wanted?
• I think the most important for me is playlist control: reverse button for playing videos from newest to oldest and play/not next video from playlist (sometime I not want to automatically jump to the next video, I relisten some music twise or triple times before I think I can switch to the text video, manually (actually I can toggle button for automatical switching work if I need focusing on something else)).
• The second important for me is seeing how many videos this channel has right after nickname. This feauture compleately lost from some old youtube design and I very miss it.
Another features I use frequently:
• watching videos in search or channel in grid (compact) view (and using elipsis in video names to short name, unless I hover on it),
• hiding comment section in a spoiler (it's a wonderful passive ability, cpmments not loaded until I really them to read, because I mostly use youtube for music and scroll down to check desciption only and I no need stupid comments at all),
• drawing screenshot from the video moment to canvas for futore copy and paste in discord or saving as image,
• with the above hide and seek frame by frame by using Shift + Arrow Left/Right for get exactly frame I need,
• popping up video in new minimal window, I also use autohotkey for removing borders, make it always on top and position in one place. With that I place some MV's in the special area and coding in my IDE, while watching video, I really lost this ability without external tools.
• also feauture for opening videos section by default instead of main channel page, probably I not look at it in 99% times when I search channel for interesting content, I only need videos and playlists on the channel, so I still want to use this feauture to navigate without extra loading and waiting time (it really boring after many times),
• also used small checks for removing unnessesary panels for me, such as recommended channels (yt still sucks in recommending)

Also I will be appresiated if flowing video will be able to move (with saving position) on the screen instead of hard fixing it in one of the corner. I probably use large size of it, I hate smal size of viewpots, but the comments what may be under it sometimes I still want to read and this require for me to temporary move my vid from bottom-right corner to top temporary (after reading and may be responding I put it back to the place).

Also as you have no popup option I will be appresiated if you can add at least fake fullscreen option (video will be just adjusted on the all space without saving aspect ratio), it's like an opening embed video link what I do now to make popup windows.

Anyway in your way your addon not so bad, just for me it not so convinient and I can't use more potential from Youtube what I use with mentioned userscript, without it I spent more time to do or setup several things I do in one click before, such as popup or showing channel videos by default.