Further recommendations wanted please

Unluckily the last time I posted I did not receive enough good responses to help me make up my mind . The program that I finally decide to use has to be completely FREE as I don’t have much money to spare and also over time I will find out if this is really for me .

I am looking for a way to make extra income putting designs on products for sale (print on demand) something that is VERY easy & straightforward to use for a complete beginner.

Is an online one or one that I would have to download best e.g Inkscape – Gimp – Vectr – Canva - Gravit – Paint.net - Paint 3d but the problem I have found is that most of them seem to be far too difficult for a beginner. What I would like is a program with a reasonable learning curve but at the same time have fairly good features. I obviously can’t expect the best , but I would like something that I can learn with but is fairly easy to grasp in the beginning Or are the ones on certain websites like spreadshirt okay to use for the purpose of designing T-shirts , coffee mugs , hats etc..

All recommendations very appreciated.