merch by Amazon T-shirt designer

I need T shirts designer, with experience and knowledge on merch by Amazon.
The designs should be original and new (checked by me).
You have to do keyword research to find designs that sell.
I need some designs for trial and then work will On a monthly basis or according to the number of designs because at the moment I have an option for 100 designs.
Of course, the design rights will be mine and only mine.