I agree on aiming perks at the mumsnetter rather than at the family or "mum". If I'm paying for a subscription for myself, I want benefits that I'd enjoy for myself. Home insurance is not a treat for me, neither is money off my utilities, and I have no need for pushchairs nowadays.
Offer some premium exclusive competitions and product tests, maybe even throw in some sort of one-off subscription swag. I have a magazine subscription that costs me roughly £5 a month and with that I got a fairly decent gift at the start of the subscription and I get one again once per year when I renew it - the gift changes so I'd guess its provided by whoever is sponsoring them at the time the subscription is taken out? Audible gives me a free audiobooks each month alongside whatever audiobooks I've paid for. Chewigem gives me one free credit each month to spend in their shop, 1 item = 1 credit.
Remove all of the adverts. If I'm paying for a subscription, I don't want to see a single one. Removing some or most isn't enough.
I'd want tech features that make the cost worth it, not just the quote function but an extra level of filters such as the filter by OP option mentioned, better/more emotes, the option of choosing which username to start a thread as without having to go into my account to change usernames, if I use a name change on a thread then for whatever reason then for that thread to "remember" which username I posted with and to default to that username on that thread to prevent any name change fails (would stop people sockpuppetting too).