Flower mechanics haven't changed in a very long time, so everything in these particular guides still stands true ^^ I'm really picky about what guides I use, so I got to know the community that datamined and/or did extensive testing on this info and 100% stand behind it on accuracy. Hopefully it's helpful to you!
Comprehensive Flower FAQ: https://aiterusawato.github.io/satogu/acnh/flowers/faq.html
Garden Layout Guide: https://yuexr.github.io/acnh/garden_layouts
Misc Flower Guides (reproduction mechanics, flower phenotypes, etc.)
: https://cestislife.github.io/flower_guides
Flower Breeding Simulator: https://gardenscience.ac/