Could you perhaps print a mould and then cast platinum-cure silicone in it? This can be a pain, as lots of resins contain things that stop silicone curing, but some combinations work (e.g. some bbdino silicone I had cured just fine in contact with Sunlu ABS-like clear blue resin I had) or can be made to work (e.g. coating the resin mould with something that it's more compatible with).

If you don't need the properties of platinum-cure silicone, you might even be able to get away with tin-cure, which is much cheaper and more tolerant of other chemicals.

Formlabs sells a materials as low as 40A. Silicone 40A. Or if 50 is Ok then an Elastic 50A, or Biomed 50A.

I can't speak to if they are formulated to cure with any resin printer or only via their own.

https://formlabs.com/materials/?characteristics%5B0%5D=21#materialproductlist

What's your application, can you substitute geometry structure to provide "softness".

You need a silicone based resin. TPU based are 65A or stiffer. Stratasys or Formlabs make these down to 25A, but they are expensive. Very difficult to print most shapes as the print inherently sags. They print very slow, 30 second first layers.

Spool3D elastic is 40A and inexpensive.

https://spool3d.ca/spool3d-elastic-resin/

Voidstar Lab might know. Half his videos are about the wildest filiaments out there, and I know I've seen one or two featuring nothing but soft and squishy, rubber-like materials.