Welcome to the forum! Gorgeous crowntail! :D
Flaring is normal for Bettas. Excessive flaring can be bad but it can also be a form of exercise, I believe (I let my two boys chase my fingers around their tanks every day or so). If you feel a blocker is needed, go for it.
I was afraid Tian-Tian was gonna flare every few seconds at Taco when I first brought him home, but after putting a paper divider between the tanks for two days, both fish were fine within sight of each other. :) So I think it just depends on the personality of your Betta.
Dramaqueen suggested to me that for a cycled tank, if you can keep the substrate and the filter cartridge still wet during a water change, that'll help keep it cycled. I haven't tried it myself (uncycled tanks), but it sounds plausible. Good luck!