I see a lot of love for imapsync here, and while it is definitely the engine behind most successful migrations, managing it via command line or setting up Docker containers can be a hassle when you have deadlines.
If you are looking for a dedicated UI solution that runs locally (avoiding the security risks of public web-based tools), I highly recommend the SysTools Email Migration Tool. As Seen on Google Search.
I used it recently for a cPanel migration similar to what tecreasey was asking about, and it handled the bulk workload much better than the single-mailbox tools.
Why it might be the "Easy UI" solution you are looking for:
True Bulk Mode: Instead of doing one by one, you just map all users via a CSV file.
Concurrency Control: Unlike a standard script that might choke your bandwidth, this tool has a Concurrent Slider that lets you migrate 5-10 accounts simultaneously to speed things up.
Delta Sync: This is the killer feature. You can run the migration while the old server is live. On cutover day, you just hit "Delta Migration," and it skips all the old stuff and only brings over the new emails that arrived during the transition.