How big of a boot SSD do you think we need. If macOS and apps fit nicely in a 256 SSD, then I'll get an external 2TB external and be done with it.
Well, I started with a 256GB SSD on my 2012 CheeseGrater in 2012 ... but had to move up to a 512GB in 2017.
That 512GB boot drive is getting tight again; the guilty party is the OS doing Journaling of attached Time Machine drives (I've never stored my main data on the boot drive) and /Library/Mail/ ... can't readily (painlessly) move these off of the boot drive.
I'm debating if to upgrade to 1TB, or replace the whole machine, where I'd futureproof with a 2TB boot drive SSD.
For my lightweight use MBP laptop, I went cheap with the 256GB. Even by keeping it on a separate email account that doesn't get spam, its getting marginal.
For data, this is a huge YMMV based on one's own workflow & use case needs. I originally set up this CheeseGrater with a (2+2)TB RAID-0 using two internal bays, and the other two internal bays are still on TimeMachine backups (originally 4TB, but now a 6TB & 8TB), plus some externals for removable backups that are for going off-site.
For a 27" iMac replacement candidate, these externals would stay, plus all of my internals would have to move to externals too, with the commensurately ugly rats nest piles of of signal & power cables, TB/USB hubs, etc.
A lot of work & money ... not something to be looking forward to: I dread having to get a new Mac.