I was about to post something very similar: the degree of benefit you get from having multiple displays depends a lot on the amount of multi-tasking that you have.
If you can focus most of your time on a single window then a single monitor is just fine.
But when you have to reason across multiple windows very very often then multiple displays help a lot.
For me it’s a bit messy: i am a cloud engineer and the kind of work i do varies multiple times a day. At some point I’m writing terraform code and all i need is my editor and a shell (sometimes my editor is in my shell) while ten minutes later i might be doing incident response and then i need a multitude of windows (shell, web browser showing logs, web browser showing metrics, web browser showing the aws console, web browser showing the meeting with other people handling the incident with me, shell, other stuff)…
So yeah, it really depends.
Similar job. My solution was a single 4k monitor and Stage Manager. I can tile viewing a log and having a terminal open, and then just pop back to a browser when I need to. Plus, terminal can have tabs.
When monitors were 1024 by 768, I needed more than one monitor. Now that everything is designed to be one’s only window at 1920 by 1080, I need a 4k monitor. I imagine that when 4k becomes the default, I will need a 16k monitor.