If not able to stop using WhatsApp, how important is it to download it from the Play Store vs WhatsApp's website itself?

Albatross:

for people that can’t really not stop using WhatsApp, does installing the app via the APK of their website provide any virtual benefit over the Play Store?

If security is your concern, you should avoid the Trust on First Use model for app sourcing. While the ability to sideload APKs is a great advantage of AOSP, this doesn’t mean you should prefer website APKs over a good centralized app store. Obtainium et al. are excellent for reducing platform dependency (which might or might not be a priority of yours) and providing automatic updates where there are none, but outside of those specific use cases, that is where their benefits stop.