Do you know how your SSD is formatted? I don’t remember which version of macOS supported AFPS, but if your SSD is formatted that way, it’s pretty easy to create another volume and boot from that.
Still, it’s also not hard to add/create a partition with Disk Utility on the current boot disk if you have enough free space, even if it’s HFS+. The trick, as you say, is to be able to install the older macOS. There must be articles out there about how to do it.
Maybe this one, which lists several methods.
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