How to keep framerate but increase video duration with ffmpeg minterpolate?
I also asked this question on reddit and got a good answer, so I'll paste it here for maximum future search-ability of this problem:

minterpolate has multiple modes:

Duplicate frames as necessary
Blend frames as necessary
Actually calculate motion vectors and perform interpolation

The third option is the highest quality (if it works correctly, which
it doesn't for me, always produces weird artifacts) but it's
incredibly slow. Definitely not possible in less than 24 hours. So
option two is probably what you're after. That being said, 10,000
frames in 10 hours (36,000 seconds) equals a raw framerate of 10/36
fps (0.2777778), so I don't think frame blending will look very good,
but I guess you could try (I'm assuming your animation is stored in a
series of image files):
ffmpeg -f image2 -framerate 10/36 -i &quot;frame_%05d.png&quot; -filter_complex &quot;[0:V:0]minterpolate=fps=25/1:mi_mode=blend[out]&quot; -map &quot;[out]&quot; -c:v libx264 -pix_fmt yuv420p -crf 20 -f mp4 -movflags +faststart &quot;output.mp4&quot;