Spotlight

Frame animation

A strip of stills and clips, each held for as long as you say, joined by transitions and written out as one MP4 or one GIF.

One of the tools in Aspects for macOS. $19 once, cloud optional.

A sequence, not a set
Each cell owns its own file and its own duration, so reordering is dragging a cell rather than renaming anything on disk.
Mix stills and clips
A photograph next to a video is the normal case here. Every frame carries its own flag, so the two never have to be sorted apart.
Transitions sit between
A frame set to one second is on screen for a whole second and the transition is extra, so the duration you typed is the duration you get.
Two ways out
MP4 samples the timeline at a fixed rate. GIF walks the segments instead and stores a delay per frame, so a three second hold is one frame.
Per frame or all at once
Set a duration on one cell, or apply it to every cell in the strip. Include and exclude frames without removing them from the sequence.
Output size
Thirteen ratio presets or a custom W by H, with a frame rate and a compression preset for the MP4 and a loop switch for the GIF.