Exchange Online (hybrid vs. cloud conundrum)

Im not sure how to proceed with a deployment of Exchange Online.

I have an on-prem forest with two sub domains. Years ago, this was a shared exchange on-prem org. Sub domain A.contoso.local migrated to gmail. Sub domain B.contoso.local migrated to hybrid exchange online.

Now, years later, A.contoso.local wants to migrate from gmail to exchange online cloud, not hybrid. A.contoso.local user attributes are a mess, there are some users with a targetAddress of SMTP:user@A.contoso.local, some with proxyAddresses of SMTP:user@A.contoso.local, smtp:useralias@A.contoso.local, some have various msExch* attributes, while some have one or none of those attributes. All users have their mail attribute set appropriately. Entra ID Connect has been syncing for years but only recently has a license for Exchange online been provisioned, initially for just a few users. Those users mailboxes were created automatically as a result of assigning the license. Where thigs start getting weird is with Contacts in the Exchange online admin portal. Only some of the entra users show up there, maybe only 2/3's of them, they show as ContactType = MailUser. Actual Contacts sync fine from on-prem to Contacts. Contacts show up fine in the GAL too. Also strange that when the on-prem users proxyAddresses attributes change, they are not updating in EntraID. Sync works fine on other attributes otherwise.

We arent quite ready to migrate the gmail users to exchange online, first we are rolling out Teams. We want to get the Contacts \ GAL working correctly so that people in Teams can use it as the source of truth. But like mentioned, only 2/3 of the users are there in the GAL. Even then, none of them show up in the Teams People app unless you search for them explicitly. No, they arent flagged as excluded from the addressbook.

Considering we eventually want to be Cloud, not hybrid, do we need to clean up any of the old msExch* attributes? Manually manage them? Why arent those attributes syncing properly through entra id connect sync? Why dont GAL users show up in Teams unless searched explicelty? Why dont Contacts show up at all in teams when they show fine in the GAL in Outlook online just fine? Sorry, lots of questions I know. Hopefully someone can help clarify any of this.