these things always come back to bite you in the back, don't they?
I never did, because I never was brave enough to reset my router and ditch the entware installation. The main reason I never did is the crazy release schedule that flexget has. They release almost every day, and the changelog is quite chaotic, and even if it wasn't, I can't be bothered to read it every day to see if the changes they introduced break my setup. That's why I won't update flexget until it stops working, as every time I do, it breaks.
Regarding the problems I had back in the day, they were caused by python3 not being available in entware, but now it is, so I guess that it shouldn't be that difficult to install. Also, given the previous reason, the steps I took back in the day might not work now, because of changes in flexget or the packages included in entware-ng.
My post included a rough step by step, although it doesn't list the commands one by one, I think you will be able to replicate it. Do you need help with a particular thing (installing python packages or creating a virtualenv?) if so, I can try to help you with that, but I'm afraid of resetting everything just to write a guide that might not work in a month, and then screwing up my whole setup.