I want to know how you had Leopard installed on a mid-2012 Macbook. Apple doesn't let you downgrade the OS to an older version than what the Mac originally shipped with, and Leopard was obsolete a long time by then. The mid-2012 should have come with Lion. The mid-2009 was the last Macbook to ship with Leopard.
I have a similar issue with trying to upgrade from El Capitan to High Sierra. Was the change between HFS and the 64 bit Apple file system a factor for you? I had El Capitan on my main boot partition and Lion on another partition and I have Disk Utility issues because Lion is using a different file system than El Capitan.
I also think virtual memory may be a factor because I don't shut down my 2012 imac often enough.
I'm insterested in this story. OP probably won't be back, huh?
You maybe have a company budget and don't know how painful it is to be poor without a salary and watch Apple and Adobe price out poor beginners. It's been 13 years since I bought Master Collection on CS3 and it works as long as Lion runs without a monthly cost but I haven't made any money or gotten a salaried job and just need it to create my own branding graphics. There are starting to be good Adobe alternatives like Amadine in comparison to Illustrator. I already know how to use Illustrator and haven't learned Amadine yet. When you buy tools, you want to keep them and not pay exponentially more when they need to be replaced.
No company budget nor am I working. The library has excellent resources as well.
Yes, always wise to buy good tools. Longevity can be relative when it comes to computers. I still have an iPhone 6 and my wife still uses a nine year old Samsung. Go Samsung (installed SSD, creaky lid)!