I am not the pleco expert as others on this forum are, however I have had some success over the past 3 months with two varieties of these fish and I am growing to like them more and more.
From what you write it sounds like you have things pretty much right. I have found plecos like wood and the floating plants. You can add a cave but so far I have not found this essential. I feed my young plecos blanched zucchini and other dense vegetables, such as cucumber, yam and carrots. The blanching helps breakdown the plant fiber a bit as well as helps the food sink to the bottom where the plecos can get at it. In addition to feeding the vegetables I supplement with algae sticks which breakdown pretty quickly and the young plecos love it. Because I am having some success I am also looking into making my own DIY gel food.
Since it's best for you not to set-up another tank ... you might consider at pretty low cost an external breeder box that has flow through water filtering. More than likely if you have had one BNP spawn, there will be others. I use the Marina type but I think the Sudo and Finnex both look like they will easily get the job done.
Air driven - Marina
View attachment 141250
Air driven - Sudo
View attachment 141258
Powerhead driven - Finnex
Personally I like the air driven boxes ... one less plug-in and more than likely you are running an air pump so probably have extra air supply if you are only running the one tank.