From png to tmx and then import to Construct

I don't know why do you need to do that. If you just want to make levels you should use traditional pipeline with tile editor. Tilemap is an optimisation functionality. If your tiles are unique you should use sprite instead.

Do you think it's possible to go from png to tmx? first I convert a png file to a txt file with Image Magick, then I convert the txt file to a tmx file with an online converter. then I import the tmx file to C2/C3 as a tilemap. do you think it could work?

You can use Shoebox for this task: renderhjs.net/shoebox