Much to my chagrin, we have a number of supported machines which will continue to run versions from El Capitan through High Sierra and even Mojave – and a couple instances of Snow Leopard (Server) – in a VM, just so access to many dated, but impossible/difficult (e.g., Quicktime 7) and bothersome (e.g., iWeb, etc.) to replace apps can continue to live past their best-by dates.
Both Fusion and Parallels make sharing data in common Shared and User folders seamless and painless, so other than needing adequate RAM for the VM and plunking down the bucks for a novice-friendly interface (I feel the open source options are just too ugly for nervous users), they are totally viable.
As stated above, you can clone your existing functioning 32/64-bit macOS installation, and drop it into a VM intact; but I would caution that environment might have a number of background apps and tasks running that would be unnecessary duplicates in a VM, so you’d want to police the Startup and Login items to make sure you’re not running things there you don’t need (e.g, Adobe, Chrome, et al, Updaters; iStat Menus, Memory Clean, etc) inside the VM.
As for clean installs, don’t let the myths lead you down a painful path; it’s *NIX, not Windows; it will only load what you (and your app’s installers) tell it to load, and nothing else; it doesn’t have a Registry filled with dead .dlls and countless conflicts when something dies.
I am literally running the same Mac OS to macOS installation started with the original release (and I believe even from the OS X Public Beta); there are zero measurable, let alone perceivable performance differences between it and a virgin installation of macOS and like applications. It has migrated across several Macs, as has most all the installations in our groups.
With rare exception, when a new Mac is acquired, we simply transfer a clone to the new Mac and keep working with little interruption. The only exception is when a new Mac ships with an as-of-yet-unreleased version of macOS specific to that hardware, which these days, is exceedingly rare.
It’s my belief that people who espouse a regular or periodical fresh installation of macOS and all your apps, and then often a manual migration of data (i.e., not even trusting Migration Assistant), are often coming from the Windows World, where this practice absolutely is beneficial and frequently necessary; that they are perhaps reflecting on days when wiping an HDD and starting clean, rather than just pruning and defragging a disk would meet the same end, and has little impact on today’s SSDs; or that they have so few apps they can do this in just a few hours; or they are natural born tinkerers, who gain great satisfaction working on their computers, rather than with them.
The only thing you might need to be afraid of with your aging Mac is a lack of adequate RAM; I try to push anyone who does anything that pushes RAM (Safari, Photos, Fantastical 2, etc.) or likes to have multiple apps open at once, to run at a bare minimum 8GB starting with macOS Sierra, and preferably 12GB-16GB is you’re going to also run a VM and expect to use both environments simultaneously. You can do it with less, but even with an SSD, it will be horribly, horribly slow.
Hopefully your iMac already has enough RAM and a good SSD; if not, look to invest there before you worry about moving to Mojave and beyond; if your Mac is too old for those upgrades, either stay put and don’t worry about the latest macOS, or find the money for a new or new-enough used Mac.
Even if you can find replacements for all your current 32 bit apps, the money is likely better spent on hardware upgrades and running your preferred/current apps in a VM. Or not. You’ll have to do that math.
mpainesyd: