When making ouput to image sequence in other models its easy to stop and resume. We just change starting frame and going forward.
But with SlowMo its different.
We process some frames but get more image files if frame rate is increased.
If we change starting frame after stop to continue, file names numbering will not continue, but will start from frame number selected.
This makes a problem. So to join results we need to start second processing a little before first was ended (looking by image\scene content). Then we need to find same images to find place where we need to make join. And at last we need to rename all images, processed second time.
We need a more easy approach. Better built in.
I suggest that you add a button “Continue” that will appear when output to image sequence is selected and output folder specified. And that it will read the last saved image number from folder (not from save.json), calculate the frame number from which to continue and start a little before it if necessary.