I have never had that synesthetic experience thou didst talk about, but it definitely sounds great
For the first question of the thread, I recommend thee to increase fibber intake in thy diet. For example, I love eating fruits, specially apples and bananas, the later allowing me to make a big mess when I increase their intake one or two days before doing it.
Another thing thou couldst try is passing gas slowly, I get to mess and feel it that way too and for a longer period of time. Keeping muscle tension and thy butt cheeks opened can also help.
And finally, I know some hand creams have glycerin, so they have a soft laxative power. I recall I picked a little bit with the tip of my finger and introduced it inside me twice.
The first time I did it, I waited from 15 to 30 minutes, and when I give it a gentle push, I felt my body pushing intensely!
I recall I pushed twice and it felt incredible
Maybe I hadn't feel something that way since I was a kid.
The second time I did that, I ended letting all out in a single push... and it burned momentarily, so it wasn't as satisfactory. That, along with more concern about my health, made me abandon that idea.
I have to say I hadn't poop in diapers as an adult yet when I did that, however, I started experimenting with pooping and I have some anecdotes to talk about. For example, I pressed my butt cheeks to prevent poop from coming out and make it last a bit longer.
Later on, I started getting a bit out between them and I liked it, so eventually, I started messing a bit on my underwear, but filled with toilet paper. From time to time, I felt guilty and fell into purge stages.
However, one day, when I felt very sad and alone, specially referring to a crappy and utilitarian old "friend", I thought "what the heck? I'm sad anyway and nobody looks at me, who cares?", I went for real diapers and, as soon as I sat down at the bus with one on, I fell in love with them.
That's a bit of my background story, my experiences and some tips. Ammmm... I hope it has been useful for thee, heeheeheeh