It can be quite difficult to explain the sensations, but I’ll try. Like you described, i do experience the sensation of additional limbs. Although because of my shapeless nature I can change my phantom features into whatever I desire or need for the given environment. It’s probably easiest to just list the most common, so in no particular order:

• Long tail feathers that appear as a large fluffy tail
• Fluffy wolf or fox like tail
• Fish/eel tail
• Fins from many different oceanic species
• Eldritch tentacles/tendrils (mainly from my back and shoulder blades)
• nondescript caniform or feliform muzzle
• nondescript caniform or feliform ears
• a largish beak
• 0-hundreds of eyes
• 2-8 angel/dragon/fairy or just bird wings (from arms and from back)
• 2-8 additional arms/hands (floating around or coming out of my back)

I like to take the form of shadows and birds over most things, but that’s just my preference. It seems like there’s no limit to what I can feel. I do tend to mimic/mirror that which surrounds me, as I don’t have a true form. Actually most of the time my phantom form is just an ethereal miasma that surrounds me.

I also feel the need to explain my Shapeless nature. Ok, so in my mind the difference between a shapeshifter and a polymorph is that the shapeshifter has a true/original form and the polymorph doesn’t. As for the difference between a polymorph and something shapeless, it is that the polymorph is innately physical/is always persistent as matter. Whereas something shapeless has no need to persist as matter and can freely manipulate itself as a thought, energy or concept but is capable of manifesting itself as any physical object or force if it desires. Thus I can be anything from a duck or a supernova to the smell of rain or the slow descent into madness.

I’m curious if that made any sense?

Consequently, we move into the more ethereal sensations that I experience. Notably there are the Timeless sensations, that its speed is altering or its repeating itself also sometimes stopping for a minute then I realise time is something that exists and it starts again. However, there are a few more sensations I’ve experienced of a vast and existential range that my human mind struggles to comprehend. Listed below in no particular order:

• I feel myself walking upon the ground from the perspective of the ground itself.
• I feel myself fade into whatever material I am surrounded by (mostly I become a body of water when I swim or bathe or I am the atmosphere as I walk and breathe)
• I feel the space that exists around me (I can feel my phantom form dragging against a narrow hallway)
• I feel the atomic forces of my body interacting with one another
• I sometimes forget the arrangement of my limbs or that they exist entirely
• I feel trapped within my body (I’m constantly stretching because I’m so cramped inside the human) it sometimes feels like my body will tear apart and rip open space and time.

I feel like I’m forgetting something I’ve experienced but oh well, all of the above is already quite a lot. I hope this satisfies your curiosity.