I’ve written a few academic papers on my own time after grad school. It wasn’t much harder than writing papers while in school, it just took some nights & weekends time, and I had to work a little to find people willing and capable of reviewing the papers and giving feedback. It cost a bit of personal money to present at a conference whenever a paper was accepted, but it was what I wanted to do so it never bothered me.

I’m not sure I understand the question though, what seems like the hardest part of writing, what is stopping you from doing it now? Lots and lots of people write in their spare time, and there are plenty who write on academic topics and submit papers to journals and conferences. Is it finding the time, or doing the research? Being motivated, or fear about being accepted as an academic while not part of an academic institution?

FWIW, you can certainly research & write on academic topics without trying to publish in journals. Plenty of people have blogs with articles that could be journal papers if they had only a splash of formality. Lots of people are more interested in doing the reading & writing than they are in publishing. Quite a few people like that here on HN.