OK, so I love this system, I'm gonna go over some points that I think others have said and some of my own.
SO, I think the idea of the Mass thing being more so a skill issue than a hard limit is a great idea, I don't think it should be upto a nebulous connection or skill with the magic or whatever. To me, this system is really mechanised in the sense that it is all about that mask and carving it, afterwards, there isn't a lot one can do to change the magic in a mask without editting it. So I think for the runes/aons/glyphs, they should describe the animal that the mask is being bound to, so a tiger will have glyphs for Cat, Clawed, Big and Predator, idk but something like that. I also think that the size of the runes should matter but not for descriptor runes but for another set of runes which I propose, Augmentor Runes.
Augmentor Runes work like the other magic systems of Sel, as in they can be chained together and get incredibly complex but with Shifting, there really isn't a big benefit from going super complicated but making an Augmentor rune that is bigger makes it channel more Dor and makes that attribute greater. So a few examples of Augmentor Runes are Strong, Light, Tough, Fire, Light, etc. So each Augmentor Rune would be connected to one of the Descriptor Runes to augment that attribute. It it very dependent on Intent and picturing your outcome. When carving an Augmentor Rune, you need to picture what the Augmentation of Fire would do and so on. This allows the system to be more magical in a sense and allows for some scaling. Btw Augmentor Runes aren't beginner stuff or even intermediate stuff, they are for legit masters of Shifting.
ONTO the Mass Rule, as others have stated it is severely limiting so I propose a Realmatic Rune that lets you do a Mass to Investiture and back conversion but I also say that the Rune works the same as an Augmentor Rune meaning the bigger it is, the more Mass can be transferred so if you're trying to become like a hamster, it'll be a huge rune so you need the skill to carve everything else with the remaining space on the mask. It's the same rune for smaller creatures and bigger.
I love the sacrificial ritual thing, I think that in terms of realmatics, you are storing the Spiritweb of the creature in the mask and accessing it through the Connection between you and the mask that was forged during the Carving Process. Then there is flow of investiture with Intent which is just Invested Healing but the Mask makes it so that rather than filtering your Spiritweb to the physical, it filters the animal's spiritweb instead. I don't know how accurate this is but that's the vibe I got from it.
Also for the distance from place problem, I don't think it's an appreciable issue for Shifting, it isn't a very Investiture intensive system and the power issue of further from place of origin isn't as Intense with Shifting so you can go to the other side of the planet from your place and it will definitely lock you out of some of the more investiture intensive forms but you still have access to a lot of your beginner forms, basically distance makes the Realmatic and Augmentor Runes work less and less so the basics would work but you would be a Giant Hamster instead of a little guy.
For the mask itself, I think it must be carved by you and with the intent to be a Shifting Mask. I don't think this is a magic system that can be easily Unkeyed because it isn't an issue with Identity but rather Connection as the carving process forms a connection between the Mask and you.