My preference leans strongly toward the convenience of my MacBook for editing over a desktop computer. I am no longer using a desktop. The portability is a big advantage. I can take the MacBook with me when traveling if I desire, and easily edit some of my photos during the trip. When at home, I can edit from the comfort of the chair I prefer when using the MacBook for editing. However, I do believe it is a matter of personal preference. If you feel you really need the larger 24" display to adequately view and edit your photos, that may work better for you. Otherwise, I would go with the MacBook.

My (win10) desktop died about 2 years ago. I limped along on my (win10) laptop (still using it) but then got an Apple Silicon M1 laptop. The old laptop now does the residual windows applications that Apple doesn't support and the new Mac laptop does everything else. I got the 16" screen, which is not quite up to the 21" screen that I had but I moved to smaller quarters also and don't have room to attach the 21" screen to the Mac. When I get more space I will add the 21" screen and have a dual screen system, which I had on the desktop previously. It helps to be able to run both LR and PS on different screens.

You might find the new Apple Silicon laptop to be sufficient for all your needs. When you need a desktop, just put the laptop down and connect the extra monitor.

The answer is no. This used to be a capability, at least with early iMacs such as yours, but the ability to do so was removed from the more recent operating systems. First, later model iMacs did not support being used as external monitors, and then they just got rid of the whole thing.

See this article: https://support.apple.com/en-us/105126

This is one of the reasons I don't like iMacs anymore. Sure, they were a relatively inexpensive way to get a computer, but when you get rid of an out of date computer, you have to get rid of a perfectly good and very nice monitor. I had a Late 2015 27-inch iMac Retina 5K. Great monitor, but eventually the MacOS didn't support the computer and I sold it.

And the Mac Mini has solved that issue. I do wish memory and storage was upgradable, so you need to get as much as you can afford when you buy one. However, at least with the latest thunderbolt interfaces, you can get very fast storage; I use an OWC external that stacks on the Mini, provides thunderbolt expansion, has an SSD slot, and a 3.5" hard drive slot. I put a 4TB SSD and 12TB hard drive in it.

My iMac is late 2014 and will not update past Big Sur v.11.7.10. Therefore, I can no longer update Lightroom Classic nor use the new features. I also have a MacBook Pro (2016) and it will update to Monterey v.12.7.2 Lightroom is version 13.1 on the MacBook and v.12.5 on the iMac. I can use the new blur feature on the MacBook; but when I open the same picture in the same catalog on the iMac, the effects are not there. I will need to update one of my computers, but which one?

I know Adobe recommends macOS Ventura v.13 and an XDR display. The newest MacBook Pro has a Liquid Retina XDR display but the newest iMAC has a (plain?) Liquid Retina display. Currently, I prefer to edit my photos on the 24-inch screen of the iMAC but I can edit on the MacBook even though the display is smaller. Unfortunately, when comparing a new MacBook vs a new iMAC, you can't compare apples to apples (pun intended). The chip is better in the MacBook but you can get more unified memory for $370 less in the iMac. What is more important? The chip or the memory? Just for clarity, I've included the specs for my 2 choices. Which would you choose? Pease don't try to talk me out of an Apple computer. I HATE the Microsoft OS.

iMAC 24" - $2329.00 (-$120 for 8-core CPU, 8-core GPU and no USB 3 ports)
M3 chip with 8-core CPU, 10-core GPU, 16-core Neural Engine
24 GB unified memory (-$200 for 16 GB, but will 16 GB be enough?)
1 TB SSD
2 Thunderbolt 4 ports
2 USB 3 ports
Magic Mouse
Keyboard with numeric pad

MacBook Pro 16"- $2699.00
M3 Pro chip with 12-core CPU, 18-core GPU, 16-core Neural Engine
18 GB unified memory (+$400 for 36 GB)
1 TB SSD
MagSafe 3 port for power
3 Thunderbolt 4 ports
HDMI port
SDXC card slot
headphone jack
140 W USB-C Power adaptor

Okay, I was in a very similar situation with my 2017 iMac and had to make a decision and I, like you, will never go back to Microsoft. So, after some chatting with my computer guru who has performed some upgrades on my Macs in the past and visiting the Apple store and looking at what was offered. I decided to go with the Mac Studio Ultra and the Apple Studio Display. I'm 68yrs. old and I'm also old school in that I hate having to change/upgrade computers very 5 to 7 years. So what I decided to do is make a purchase that would be well more than I needed to edit my photos and would buy more years of service. Again, this is a monster computer and display that is very expensive; however, my photography is my passion and worth every dollar spent on the editing equipment. Now that I have had and used the Mac Studio Ultra for about a week, I can tell you that it is by far nothing like anything I have ever used before and I have had three iMacs and two Macbook Pros. If you can afford this setup, I would HIGHLY recommend getting it.....I assure you, you won't regret it. You are more than welcome to contact me at any time if you have questions.