I recommend Paint.NET with the Seamless Helper Plugin (direct download). After installation of Paint.NET, you need to copy that DLL file to C:\Program Files\paint.net\Effects\SeamlessHelper.dll.
Here's how easy it is:
Create a new picture with even pixel length. I use 200x200 px here.
Draw something on the center horizontal and vertical lines, like so:
IrfanView is a free image-editing tool with a many batch-operation options, as shown below, and the JPEG save dialog includes the progressive option.
It is a Windows application, but runs well under wine in Linux (and is available in a Snap port), so it might work in wine on MacOS, also.
The only possible disadvantage is the large number of choices, but ...
There are options to produce a painted effect to an image in G'MIC (this can be either standalone or a GIMP plug-in).
Both are free, gratis & open source, and available for Windows, OS-X, Linux and other platforms.
Example of the G'MIC watercolour in action: