I have tested this by changing a text file (but keeping it the same file size) on my computer and then trying to sync it to FTP. I think by default this file is skipped. Am I mistaken?
The flag --update does half of what I want (it transfers files that are newer even if the file sizes matches) but, according to documentation, half of what I don't want (it skips all files that are older, even if the file size is different).
Modtime support is generally weak across FTP servers due to ancient default format of listings and because standard protocol has NO way to change modtime of a file. FTP is ancient and limited.
We have IDEAS on how to improve situation for SOME ftp servers using non-standard protocol extensions. Probably some of them will be released this year. Even then they will require additional configuration in rclone.
In the meantime I recommend you use SFTP. It supports modtime out of the box. In most cases it supports hashsums out of the box. It's modern. Use it.