For all intents and purposes if the specs meet your needs a 16" MBP with the LG 5K display will feel pretty much like an iMac, except you can unplug the monitor and take the computer with you. That's how I work, I hate being tied to one specific location for everything I need to do on a computer.
I have that setup too, and actually came from a late 2014 iMac 5k. As far as I can tell, LG Ultrafine 5k is the same display. It behaves just like what I had. Including auto-brightness (and webcam, speakers, mic, but I don't use those). The 2014 panel started to have pretty bad ghosting, if any window was still, even for a minute, so it was time for something new.
Since I've got an occasional need for a powerful laptop with a large display, and a constant need for a powerful desktop with a color-accurate display, I thought this was the most economic way to get up to date with both of those needs.
Now I have the portability option without keeping two different Macs in sync. And the MBP is faster in many operations. It produces more fan noise, but that I keep in check with Turbo Boost Switcher.
The two side monitors are ThinkVision 24" (2560 x 1440). Together the three are mounted on a Mozi triple display stand. LG uses a direct Thunderbolt connection, and ThinkVisions are connected through Caldigit Mini TB3, dual HDMI. The MBP is in clamshell mode with two wires attached. It's been a reliable setup so far, including Windows gaming (Boot Camp). I might upgrade the side monitors to 24" 4k in the coming 2-3 years.
For external storage I went all SSD. Samsung T5 runs at 350 MB/s, and I've mounted two of those behind the LG, connected to its USB-C 3.1 gen 1 ports. It's invisible, fast and silent. With Turbo Boost off, there's still plenty of overall performance and almost zero sound. No More Nails was perfect for attaching those 50 gram drives as a stack.
