I went with top pre-built tier (fast CPU/GPU + 2TB Fusion) as well, received yesterday. I already have a company-supplied work laptop (Windows), Sandy Bridge-era gaming rig (+ 980ti), and a 12” rMB 2015 as my sole MacOS device.
I picked up an iPad Pro 10.5 a few months back as a compliment to the rather behemoth work laptop. As a complimentary device it works way better for me than the 12” rMB: smaller/lighter, faster for photos and ebooks (iBooks on MacOS still seems to get bogged down very quickly when searching large books), better battery life, personal laptop stand-in (with smart keyboard cover), notepad (with apple pencil), ebook reader (true tone), web browsing, movies when traveling, and casual gaming. Really a good jack-of-all-trades secondary device for both business and personal use.
As a personal desktop, main use cases these days are processing photos, a bit of gaming (though I’ve moved mostly back to console for that, I’m not a competitive gamer), and general programming/scripting. The iMac is far better in that role, planning on selling both the 12” rMB and possibly the gaming rig. For very large storage and media I already have a dedicated DIY NAS server, however I wanted a large enough internal drive to pull full versions of Photos and all iCloud Drive files for local backup and 3rd party cloud backup... for that use case the 2TB Fusion drive is ideal: relatively small fast working set on the 128GB ssd and rarely touched large file storage, that is stored local for backup purposes.