DVD's max out at about 580p (for PAL, NTSC is 480p), resolutions are measured by the number of horizontal lines of pixels (counted from top to bottom of video/screen), not vertical which at 4:3 square aspect ratio on dvds does tend to be 720 pixels (by contrast full resolution HD video's number of vertical lines is 1920 while it's horizontal lines are of course 1080, hence 1080p). You're not the first person to be confused by this.

Professional encoders who fully understand the encoders and the schemes in use and care about not seeing artifacting or low quality would never intentionally go as low as 300mb for a feature length movie of even an hour. Yes there are people who do such things but they're not well regarded and it won't look even passable on anything larger than a phone screen.

Recognized quality groups that seek low sizes might get an animated feature (less bitrate needed due to lack of fine detail in animation vs real film) in SD quality down to around that. But for most live action content the sizes I see from the best of the best concerned with smaller release sizes are in the 900mb to 1.5GB range for 60-90 minute features.

300mb for a 90 minute live action feature even in SD is just not going to look good, some of the groups who get those sizes make them look even half-passable by running pre-filters in virtualdub that smooth, reduce grain and detail, etc before passing to the encoder. That kind of thing is way beyond anything you're going to learn in a few youtube videos though, that's advanced stuff with scripting.

Think about it this way, if you shoot for 1GB encodes with 265 or AV1 you can store over 900 movies on a 1tb drive which can be had for well under a hundred dollars.

I would like the best and fanciest algorithms to have least dataloss.

There is no magic that will get you where you want. If you want detail preserved you need more bitrate which translates to larger sizes. Modern codecs like HEVC and AV1 mean you need as much as 1/5th the bitrate you needed with old MPEG2/4 encoding schemes used on DVDs, that's darn good savings but it has its limits.

Do as you will but anything live action (non-animated) significantly under 1000kbps average bitrate is going to look awful on a 1080p screen and much worse than what it would look like if you popped your dvd in the disc drive and played it from there.

Opus is fine if you're not worried about compatibility and just playing on a computer.