Probably not considered simple, but the free tier of DaVinci Resolve is excellent and can make custom dimensioned videos. I produce a lot of videos in a variety

Probably not considered simple, but the free tier of DaVinci Resolve is excellent and can make custom dimensioned videos. I produce a lot of videos in a variety of proportions (including 4x5 and 9x16) and it works well. Downside is that the interface is best for landscape videos; I always wish I could rearrange the interface to accommodate a larger vertical preview.