Personally, I do not like cloning drives. I never saw or rather I should say HAD the need to do so. Perhaps it's just my own little quirk, but then again, I do reinstall Windows every 6 -8 months to ensure everything is running at peak. I'm sure some/many here will disagree. To each their own.
As for it being a boot drive, that's the whole point. You can't boot from an empty drive. Even if you clone your HDD over to the M.2 successfully, you still have potential issues inherent with HDD's, which are vastly inferior to SSD's transferring over as well during the process. An HDD is like an office. No matter how organized you are, files still go missing or have coffee spilled on them corrupting them etc. Then what if you have a frenemy that is jealous of your success and comes over and places a bug under the table, you know, something right out James Bond. This is malware. Then lets say you also go to websites you shouldn't and "catch a cold" that will then transfer over to your new m.2. Unless you are in the middle of making the next block buster film for say Pixar and have no other choice but to clone, then do it. Otherwise, and again, this is just me and my own personal opinion, if you get a new primary drive. Install a fresh version of windows. It really does not take that long and the benefits outweigh the negatives.
There are of course variables that come into play, such as what if you don't have a USB drive with Win10 or no internet at the moment to download drivers or a slow internet or metered connection and don't want to spend 50hrs re-downloading everything.. Or any other number of root causes why cloning may be the better option.
I'll let others offer their suggestions so that you can better make an informed decision, as my way isn't necessarily the correct way! Haha..
Damn so I just re-read your last post and I think I totally missed your point.. Ha ha.. You can have both the HDD and the SSD installed. You'll continue to boot from your HDD and can use the SSD as storage. If you want to use the SSD as a boot drive, you'll need to install Windows/Linux or clone. My rant above still stands tho. lol..