OP, it's not black and white though. You can consider keeping DD in nursery when your DC2 arrives for one day a week, or even half a day if possible, to give her some time to socialise and give you time with DC2. It doesn't have to be all or nothing.
Those who say take mortgage holiday - please don't. In 6 months time you'll be hit with much higher repayments. That's just poor financial advice and planning.
Try to sit down and think about it strategically. You'll have to bear the cost of childcare anyway if you're hoping for more than one child. The fact you are having them closer together means one thing - these costs are piled up at once and not spread out. Nothing else changes. There is a positive in you having much more of a breather once this expensive period is over. So for the time being you need to do financially what you'd do anyway - spread the cost of childcare over a longer period of time. Personal loans can have various interest and lending criteria, from experience Halifax tends to offer 3.5% interest to customers and it would likely be that for you on a say 15-20k loan. That was my personal loan for a car and it really doesn't work out much monthly over 5 years and not much in interest tbh. They lend up to 7 years. You need to compare the loans, I think on one of the main money saving websites you can do this. Most banks also offer a soft check to see if you are likely to be accepted, before runing a hard credit check. Shop around. In 5 years' time or less you won't even know you had a loan and at the time both your DCs will be in school and you'll be on better money and saving.
I'd also get a credit card (whoever is not taking the loan) with 0.0% on purchases and bslance transfers if you have anything to transfer, as a back up to those unpredicted rainy day purchases.
I've had to go through something similar before with house renovation. Now I'm going through having twice as big of a mortgage, renovation and baby on the way, due this summer. FWIW, I had 3 MCs and it was never planned that all these things would come together at once. Both DH and I are paid well, much better than before, but it's still incredibly hard and we know with all this going on we're in for a 5 year tight run, probably no extras and not much money spare. But with savvy financial management, ie cutting down, savings and sensible loans, we will go through it. I too like you tend to plan on the current money for years to come. However, please don't forget that money fluctuates and whilst of course anyone's position can get worse, it usually gets reasonably better, particularly if your DH brings in so little now. There definitely is scope for improvement there and if not, he'll need to pull the plug and go out to make more money elsewhere. We all have to think this way.
Re HG - you may not have it in every pregnancy, don't set yourself up for it automatically. If it happens, you will go through it, you have once already and that's hell of an achievement. You've got this! From that point of view, trust me there is no good time to have HG at any point in your life. Don't blame yourself and push more responsibility on DH for looking after DD if it happens. You're both in it, not just you. Any friends or family around who could help with childcare when you're poorly? Also DD will stay in nursery for now, right? Do you think you would be able to continue workinh? Can you WFH or part WFH to release the burden of commuting etc if you're poorly? If you're NHS you may get enhanced sick pay even if you can't work temporarily? It's not the end of the world and your DD will not remember you going through it, don't even start to have mum guilt over this!
A baby is a blessing if you're wishing for it, it really is. It might turn out to be so even if unplanned, after the initial shockl! Of course not always, but from what you wrote I really believe that if this pregnancy goes well for you, you will be fine, you're understandably overwhelmed now. I hope it all pans out well 💐
Get a cuppa and a piece of paper and write down your actual realistic options, research a bit and things will fall into place 💐 Wishing you best of luck, I have a lot of sympathy to how overwhelmed you're feeling now, but this will pass and you will see the way out very shortly. You will do whatever is right for you 💐 xx