TL;DR: I'm considering upgrading from the old v5 to v7, and since my setup is a bit odd I'm looking for info on what to expect. I'm also not sure of how the latest 1Password works today.
I started using 1Password for Mac a long time ago, either with version 3 or 4, which I bought on the Mac App Store.
Years later, when version 6 came out, a feature I was relying on was removed  and I had to stay on version 5. I did so following suggestions provided by Agile Bits employees on this forum (one and two), which involved downloading the old 5.4.3 version from the archive. I have been using version 5 ever since, which has worked well for my use case.
I run 1Password v5 on several macs where I'm the only user (personal, work, etc), and I've always sync'ed the 1Password vaults between them using Dropbox.
When 1Password for iOS was released I bought it too. I've kept it up to date and I've been happily using ever since. It seems to sync without issues with the agilekeychain files in my Dropbox account.
Compatibility. Would my vault be compatible with the new version? I remember reading a few years ago that the file format has changed, so now I wonder what's going to happen. Since the iOS app is working with it I suppose it's going to be fine, but I'd rather be sure. For example, would I be able to rollback? Is the upgrade a destructive process, in terms of vault storage? I'd still make a backup copy before upgrading, but it'd be good to know that I can rollback later if I prefer the old version, i.e. after modifying the vault with version 7.
Features. I'm not sure about what today is part of the "core app" and what is part of what you call "1Password Account" or "1Password Membership". (What's the difference? How come those pages are not linked on your homepage? I found them through Google.) My confusion comes from the fact that you provide instructions to migrate to a 1Password Account (see first link) and also instructions to sync vaults without it, which make me think that getting the account is optional, but at the same time your pricing model seems tied to the creation of an account, and I can't see an option to just use the app without an account. (Also see question 4)
I'm pretty sure I wouldn't use the web UI, because I prefer to just use the apps, accessing the local files. I am not sure I would use the proprietary sync either, if I can use iCloud, Dropbox, or another file-based system (see question 3). I also wouldn't share vaults with other people, but I would like if the other members of my family could use 1Password too. That would make the family plan interesting, but I don't understand if that plan is referring to the online account features or the normal app usage. In other words, is it possible today to just use the desktop mac (and iOS) application without all the sync and sharing features? What pricing plan would it be? (see also question 4)
Syncronization. I appreciate that the builtin sync provided by the 1Password account (through the Agile Bits servers?) could be handy, but my current set up works quite well for me. What I really like, I suppose, is that it gives me decent control on the process, as they're just files. (Although I understand that since version 6 the app uses an encrypted sqlite file for all the vaults, and the encrypted agilekeychain files are just sync proxies.) Is the builtin proprietary sync mandatory? (this relates to question 2) Can I still choose to use Dropbox? In terms of security, where can I find more info on how the vaults are stored?
Pricing: I was hoping to just get an updated version, and I would be happy to pay an update price, if there is one. That doesn't seem to be the case anymore, as now you're using a subscription model. I'm not against the monthly cost (I understand that it's a saner business model), but I'm not sure what I'd be getting out of it. As I stated above, I don't think I'd need the account (or membership?) features, and I just would like to use the app. This is what I find a bit confusing: based on what I've seen on the pages I linked above, that seems to be indeed possible (as getting an account is implied to be optional), but there is no pricing info for that, which makes me think that just getting the app would be free. So, can it be used for free?
Thank you for your help!
 See this old, long and closed thread for the details: https://discussions.agilebits.com/discussion/56271/individual-unlocking-of-secondary-vaults-gone-in-1password-6/p1.
1Password Version: 5.4.3
Extension Version: Not Provided
OS Version: 10.14.6
Sync Type: Dropbox