I can’t help with the hold frame except that maybe you could map keyboard shortcut to access that.
as far as the interval:
what if you make a black video clip in the project panel and set it’s duration to the interval you need….
then cut it onto track 2.
then you can quickly jump from edit to edit and in the black video and use Ctrl K to chop your source.
then you just remove the elements off track 2.
Alex