RE: Unable to import solution from On-Premise to Online.
Hey - so I'm no perfect expert either but I was almost in your exact situation, but you have even fewer users (I got about 35). I got a quote from my MSP on a migration and they came back with something like $4000 including Skykick licensing. I was basically like "Nah" and did a bunch of research on my own. Now, some would say get a partner to buy the licensing through and you will get more help with the migration for free. I bought direct from Microsoft
I eventually migrated using the native migration that Microsoft offers, using "Outlook Anywhere" to pull all the e-mails over to O365. It also pulled distribution groups, but I had to re-create public folders manually. Not a big deal since we have like 10. Our biggest mailbox had 10gb and migrated just fine. Not sure what your sizes look like.
For now I have opted to not ADsync, and will look at that again in the future.
As for our on-prem Exchange server, I buckled and will be paying our MSP to properly decommission it, as there are mixed recommendations as to how to do that to not leave junk in AD. For now it is literally just sitting turned off.
I currently use MSP mail for filtering, which then passes the e-mail to O365. However, Mimecast gets good reviews around r/sysadmin and if I weren't in a contract with MSP I'd be re-evaluating, because they don't have great syncing options.
Only "gotcha" I can think of is the internal Outlook clients still got confused and checking for the internal exchange server, even after I wiped the outlook profiles.