Best TimeMachine solution for macOS Sierra given current hardware? I have an iMac running El Capitan (can't upgrade further) that has been hosting my TimeMachine backup for years. New MacBook Pro can't see the Time Machine volume - doesn't even see the drive connected to the iMac as an option.

I have a blank 4 TB 5.25" drive I can use for an alternate TimeMachine backup. What setup should I use for optimal outcome? Can I connect it to the iMac in a way that will make it available? Enclosure connected directly to the MBP seems far less convenient than the old method, where TimeMachine would just run whenever I was on my home network...
If you buy the "Server" app from the App Store ($20 I think), it will let you once again use your iMac as a network based backup destination for Macs running Time Machine on the network. You can just turn on the Backup feature in Server.app and ignore all the rest. 

You can most likely do this without the server app using some command line tricks, but I don't know how off the top of my head. 

The only other solution is physically plugging in the drive to each Mac, one by one. But like you said, that's not ideal.