Years ago I converted my archived iMovie .dv clips to mp4. I have those also on D8/miniDV tapes and video-DVDs but nowadays it is much easier to watch H.264 or maybe H.265 mp4 everywhere. Archived original quality .dv files can be used for re-editing, if necessary.
I used the following MPEG Streamclip 1.9.3b8 (sadly the last version of this great app) preset as a base. I'd recommend doing the same unless you have good reason to use other settings. I use PAL (25fps, 720x576 rectangular pixel .dv, commonly 768x576 square pixel 4:3 mp4) but those MPEG Streamclip presets should pick the correct values for NTSC (29.97fps, 720x480 rectangular pixels, commonly 640x480 square pixel 4:3 mp4) as well as 16:9 square pixel mp4 (PAL 1024x576, NTSC 854x480).
MPEG Streamclip > File > Export to MPEG-4... > iTunes... > Apple TV 4:3 (SD) (or Apple TV 16:9 (SD) if you have widescreen project -- I guess you didn't have HD projects?).
Then choose "Deinterlace Video" (if you are 100% sure that there are NO interlaced clips, you might leave that setting off and deselect "Interlaced Scaling". But usually some clips are interlaced and you want to deinterlace them because otherwise you get awful "comb lines" with moving objects on a computer monitor).
I have found that preset's 5 Mbps Data Rate a good compromise between quality and file size (I recently did a test and higher bitrates only bloated the file size with practically no gain in quality).
Then just click "Make MP4". The output is H.264 with the original frame rate and audio sampling (48 kHz audio recommended although some iMovie versions erroneously set it to 32 kHz).
While you are at it, I'd suggest putting the date to the filename with a format like YYYY-MM-DD hh.mm.ss.mp4 or YYYY-MMDD-hhmm-ss.mp4 (my naming scheme for all images and movies).
MPEG Streamclip has also an option for batch (List > Batch List) if you have many small files that can have the same settings.
You can then use exiftool to set the movie dates so Photos.app etc sort them correctly.
https://discussions.apple.com/docs/DOC-250002750
