I have a 5-year-old ASUS M32BF desktop computer with a 2TB HDD. It came with Windows 10 installed and I have added two linux distros, first Mint 18.3 Cinnamon, and then Mint 18.3 KDE, which is now what I use 90% of the time. (I still use Windows once in a while for the odd piece of software that only runs on Windows or to use my old Canon scanner, for which there is no Linux driver. And I will probably soon add Manjaro KDE, since Mint has not kept up its KDE version.) The three OS's are spread over 9 partitions that altogether occupy about 0.75 TB on the 2 TB HDD. The largest partition, which occupies 1.25 TB, is an ntfs data partition which I keep most of my data files on so that they are easily accessible from any of linux or windows systems.

There is a spare bay in the computer and capacity to add another drive. I just purchased a 1 TB SSD which I plan to install in the spare bay. Then I want to transfer the OS's over to the SSD to speed up operations on the computer, and then keep the HDD to store the data files.
What I am hoping to do is the following: 1) install the SDD, initially as a secondary drive in the computer; 2) clone all the OS partitions, including the boot sector and grub2, from the HDD onto the SDD; 3) change the boot order in the BIOS so that the computer will boot first from the SSD; 4) once that has been checked out and everything is working properly, turn the whole 2 TB HDD over to the data.

My questions are:
1) Is this feasible, or are there issues with this plan that I have overlooked and need to work around?
2) What is the best way to clone the OS partitions from the HDD to the SSD (including the boot sector and grub2)? I have read about Clonezilla and EasyUS, but not sure which to use - or is something else better?

Advice on these matters would be very much appreciated. Thanks.

If the 1.25T partition is the last on the HD physically, would clone the whole disk to the SSD whilst booted to some live media, then reboot without HDD connected, again to the live media, to remove the remnants of the truncated NTFS partition from the SSD. Once the result is proven acceptably ready for normal operations, reboot from the live media without the SSD connected and with the HD connected to remove the OS partitions, then either expand the data partition or add an additional data partition for data using a Linux native format. Finally, boot the SSD with the HDD connected.

Rather than Manjaro, why not stick with the familiarity of Mint's Debian derivation by changing to Neon, for the most up-to-date stable KDE experience?

I use exclusively for partitioning and cloning for the past two decades DFSee, so have no basis upon which to recommend any other software for cloning.

If it shipped as Win10, it's presumably UEFI. So forget everything you've read about needing to copy the MBR - there (effectively) isn't one.
gparted apparently has a drag-and-drop type copy function for partitions - never used it but might be easiest. Not sure about how Win10 will like being moved, but worth a shot. If it all goes wrong no harm done, just start again.

I used the gparted clone function and it works just fine. Clonezilla will work as well which I use also.

I assume you're not going to want to run your older versions of Mint once you settle on your new KDE distro, whether it be Manjaro, Neon or some other. I would suggest that you install your new SSD and do an installation of the KDE distro of choice to the SSD and direct the installer to install grub to the SSD.

You can then switch back and forth between booting your old HDD and your new SSD in your bios setup. The KDE distro installer you choose will probably pick up your other installations on the HDD and make boot entries for them so you may not even have to switch boot devices in your bios but that's always a fallback if things don't go as planned. You can install several different KDE distros on your SSD to see which one you like best making sure that the installer installs grub to the SSD. Once you settle on something, delete all the linux partitions on the HDD and resize your NTFS to use the freed up space and leave windows on the HDD.

The above scenario is probably a little easier than cloning everything and getting things to boot properly afterwards. Apart from Neon, you might also want to look at Kubuntu, Mageia and OpenSuse, which are all good KDE distros. I've been running Kubuntu 20.04 which is an lts which I prefer to Neon for stability reasons. If you're into gaming, I've had problems with Neon and the 32 bit library support needed for wine,YMMV.