I installed a 1TB Feather drive in the comparable system. See some of the discussion in this thread. After I installed it, the drive would force a reboot twice any time I opened the clamshell or unplugged the power cord. After consulting tech support more recently, I had to turn off hibernation mode and something else. Now, the drive is stable, albeit with greater power demand.
I have reservations to recommend Fledging based on my experiences. They may have improved their product line in the meantime.
In short, you can increase the internal SSD to whatever size you want.
Should you change out the SSD, I also recommend to do a clean install of the OS. In my case, I also partitioned the 1TB into a 750 + 250 split. The 250 stores lots of cruft that I use to create DevonThink databases for review. You could do the same with your photo library. The approach might afford you a clearer path to do backups and archives as well.
You also mention a debate about buying a new 16in. I am on the same decision point. You may want to consider where you will send this machine should you buy the new 16in. It may be better to purchase an external SSD that can transition with you to the new machine than to buy an internal SSD that will be “given away” in a year or so.
And, should rumors prove to be true, you could always consider this option at some point in the future for less than the price of the new MBP.
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JJW