Is it possible in FFMPEG to change scale over time?
From what I understand 't' is evaluated once. Have you read this: https://ffmpeg.org/ffmpeg-filters.html#zoompan ?

Ffmpeg Docs:

9.170.1 Examples


Zoom-in up to 1.5 and pan at same time to some spot near center of picture:


zoompan=z='min(zoom+0.0015,1.5)':d=700:x='if(gte(zoom,1.5),x,x+1/a)':y='if(gte(zoom,1.5),y,y+1)':s=640x360


Zoom-in up to 1.5 and pan always at center of picture:


zoompan=z='min(zoom+0.0015,1.5)':d=700:x='iw/2-(iw/zoom/2)':y='ih/2-(ih/zoom/2)'


Same as above but without pausing:


zoompan=z='min(max(zoom,pzoom)+0.0015,1.5)':d=1:x='iw/2-(iw/zoom/2)':y='ih/2-(ih/zoom/2)'