I've been doing some google research but thought someone here would almost certainly know the answer to my question.

I've had Windows on a 250GB SSD for years now but the drive is almost always 90%+ filled.  I also have a 2TB NVME drive with about 850GB of apps and data currently installed on it.

Is it possible to create a partition on the 2TB NVME, 300GB or so, and then use a clone software to move my current c drive (Windows, etc.) onto that partition without losing the 850GB of data currently on the drive?  If so, will the partition become the new c drive after I restart and set the boot device to the NVME in bios?  Or will the partition show up as a potential boot device in bios that I can set?

I would consider a new 1TB NVME drive to replace the SSD then use that as a backup drive, 2TB would be even better. I have 3x 2TB Samsung NVME drives, having your OS on the fastest drive makes loading better.

You should be able to clone your C:\ drive and write it to the new partition on your 2TB drive. I think you need to specify the boot drive in BIOS - I would take a look at your BIOS. If that's the case, after cloning and writing to the new partition, restart into BIOS, change the boot device and you should be good.

I hate to say this but 300GB Is not big now days, everything you install will put something on your C:\ drive.

While it is possible to move your OS partition on one drive to an another drive I'd recommend against it. It is a lot of work and unless you get it right it you will end up with a system that won't boot or worse you lost data in the process.
And all that for just 300GB? Probably before this year is over you will notice that your 300GB OS partition is also at 90% capacity.
I recommend to follow @G-RFRY's advice and just purchase a larger SSD. It is the easiest solution.

If you insist on moving everything to that Nvme, the safest and often fastest method is to backup that entire Nvme drive, use third party software to clone the entire SSD to that Nvme and than move all the data from the back up back to your Nvme drive, and keep it as a single partition.

This partitioning of drives is a thing of the past IMO. You will end up with one of the partitions being too small for comfort so you start moving things around or spent too much time shrinking and enlarging partitions to make things fit.  If you have just one SSD, just keep it as a single partition. If you want to mentally separate stuff you can use folders or even the SUBST command to have a data folder appear as "D:" in your file manager.

Thanks for all the help in here.  I was able to move everything successfully.  I used Windows Disk manager to create the partition and then two freeware programs to clone the drive and merge the partitions.  I probably could have gotten away with one freeware program, but I couldn't figure out how to merge partitions with it.  So, I used a second freeware to merge the partitions.  

So far I've had numerous flights in 2020, 2024, and XP12 with no issue.  In fact, absolutely nothing is different except I now have Windows, etc. in a much bigger space.