An alternative to trying to get SS to act:
notify, refuse and block further DWP payments to your bank, and set her up with this:
www.gov.uk/payment-exception-service
She/SS then has to organize carers to pick up the money from PO or pay points (shops and supermarkets). You put whatever's in your account onto the pre-payment card hand it over and cease to have have financial arrangements with her, leaving you to decide what other level of contact you want when she calms down from it.
I have hoarding disorder (funnily, enough I never asked for it) and have done a lot of research trying to cure myself, and worked with others with it. There's many types and forms of it from organised, clean, and housed in storage, through to chaotic squalor hoarding, and everything in-between. Some people are very 'normal' outside of the condition, and range through to very eccentric.
Animal hoarding is in a category of it's own as it's almost impossible to get the person to realise they're causing harm, because they choose animals that have a narrative of 'needing saving' therefore any conditions are in their minds better than the alternative, which allows them to keep them in poor conditions and see no problem. They usually know however that others do see a problem, so a lot of mental gymnastics.
Whereas with most people who hoard items, if you do it right according to individual issues and needs, can be got to see there's some level of problem and can be helped to varying degrees.
But real change can only take place if the person wants it and it's still hard going because the stuff is the symptom of the problem, not the actual problem. The same is true of animal hoarders, but we rightly say the animals welfare needs to come first.
Trying to get rid of the symptoms without tackling what's causing them unsurprisingly just doesn't work, and people instinctively reject attempts at it. I hope hoarding Uk told you that but their 'model' didn't used to seem to recognize this.
I'm not having a go at you, there's obviously a complicated painful relationship playing out, but the reason secret (or otherwise) removal of stuff isn't recommended, is because it triggers worse accumulating and replacement accumulation. The person doesn't even have to know it's happening, just a sense that things are gone, or permanently lost is enough. One person told me the great thing about getting animals was apart from 'saving' them, her partner couldn't get rid of them without her noticing.
I'll no longer be involved with animal hoarders because I've never found a way to really help change their mindset, but it's very much about what they get from the sense of 'saving' the animal's and a sense of usefulness/ to heroic, for doing something for them. Some start out well enough and become overwhelmed, others like your mother, are totally blind to the animals needs.