YANBU to feel like this but you would be unreasonable not to sit down and discuss it with your husband. Plenty of people have felt like you and regret it. I think you need to consider -
Money. If you cant get back into your career now what will you do later? Will this cover wraparound for the 30 free hours? What are you going to do about your pension?
Future children. Do you want just one or would you consider having more? As if you have more then presumably you wouldn't go back til the eldest was in pre school and that's quite a few years out the workforce. All the financial considerations are magnified, for example you wont get any mat pay. You wouldnt be able to afford childcare for the eldest so you can have any time alone wfh the youngest. Costs of wraparound care if you do go back to work are doubled.
If you were off how would you split chores. How would you organise finances? What would your weekends look like in terms of hobbies, time off etc.
If you go back to work though make sure he is committed to being an equal parent, dont fall into the trap of being the default parent eg the only one who can settle your daughter if she is ill in the night, or the default person in charge of the house, because its very hard to do this and work full time. If you do go back he needs to do equal chores, child stuff, cooking, mental load, nursery pick ups and drop offs and taking emergency leave when your child is sick
As others have said, looking after a 6 month old baby is fun, they are engaging, they nap more reliably but still portable, you can still meet friends for coffee etc. All that will change, looking after a toddler in the winter for example when they have a cold and are clingy and tantrums etc is a lot different. You do need to be out and about all the time and doing activities. They are more messy and more frustrating than a baby.
I never felt like I wanted to be a SAHP but I was devastated to leave my baby when I went back to work. It was only after I'd been back at work a while though I realised I felt like my old self again and the weird thing was I didnt even feel like I was missing anything, it was only after a few months back I realised that I felt properly like me again and I wish I'd known that beforehand. But I realise this doesn't apply to everyone.
I'd say talk to your husband but go in with an open mind. My husband said he found being the sole earner when I was on mat leave, a lot of pressure, and he felt like the different dynamic would have changed our relationship if it was long term, and those are valid feelings. If he has genuine money worries, then listen to him and work out how you could get round that. Don't fall into the 'well I only earn x after childcare so it's not worth it's trap - remember there is pension and keeping your earning potential high to take into account (it is more likely you will take a salary cut after a few years out the workforce but your salary will rise in it) and childcare is a shared expense, a household is better off usually with 2 medium earners than one high equivalent due to tax and benefit reasons. Remember nothing is set in stone, you can go back to work and give it a few months and if you still hate it you know that you tried it and there is nothing to stop you re assessing it then. Also don't assume your work wont let you go part time - ask. If they say no then you know where you are. Again both of you dropping or condensing days may be an option. There might be other solutions you haven't thought of