Well not the solution i was looking for, but think i got timing to work.

Trying to find a lightweight subtitle editor with said features seems annoying to fiddle with; And Adjusting video and/or audio seems a problem and a pain vs working sources. Going from 23.974 something to 24 frames and rewriting timing codes seems a pain too. while i got some successes from setts, the file displayed rate, timing and joining/moving streams wasn't workable so i dropped that attempt.

So trying out -itsscale. This changes the entire video uniformly. Course we aren't interested in the video/audio changing. So by doing ffmpeg -itsscale 1.0 -i input.mkv -map 0:s -vn -an out.mkv we can adjust all subtitles at once. Problem still is how much? Well 24/23.974 = 1.0010845. Which is actually pretty close to what i wanted when glancing , Fiddling and raising the value i'm getting to 1.0015, which seems to look near perfect once i adjust the input offset a bit; Fiddling with -ss and -itsscale should let me do the job now.

i found the solution i can work with, at least for this set of videos and aligning subtitles between two different sources of the same video. Though it does involve a lot of trial and error, adjust the value, reload the subtitle and see how much it's off.

I adjust my subtitles with Subtitle Edit.

Well minor update. Found a set i an applying to all my lectures and videos that don't normally require a high bitrate.

CRF 45 looks really good, at least for slideshows. Classroom following the teacher can be ugly, especially older 360p videos, but the boards are readable. For other classroom, might bump to 35

10-12fps is also more than sufficient.

AAC VBR 0.5 is sufficient for talking (not music), also 1 channel

Finally key frames every few thousand.

Experimental, this is mostly because some of my lectures/videos are mixed with odd formats or wmv, so ffmpeg barfs when there's suddenly a format mp4 technically isn't suppose to hold when i converted from flv or wmv.

yuv410p increases compression, but not supported by AV1 (but h264 and h265 do).

Flags used: -vcodec libsvtav1 -scodec copy -map 0 -strict experimental -r 10 -acodec aac -q:a 0.5 -ac 1 -preset 5 -crf 45 -svtav1-params lookahead=120:keyint=2000

This has resulted in 60Mb for a 720 lecture in powerpoint for 80+ minutes.