Well make up your mind about how many days you want to work. Who made you doubt yourself there?
Having kids involves compromise. Look at it like this. You do what you can at the time when you can and it's the best you can do.
Try to think about the long term. In the next 5 years you will be paying a small fortune in childcare. IMO that's not the most important factor because by staying in work you will be better off long term due to increased earnings. Are you in the sort of role where you can get promoted? Do you yourself want a career where you increase earnings and seniority? Often senior positions are more flexible than junior positions.
Most women don't have enhanced mat pay. I had statutory. DH and I are still paying off debt accrued from mat leave and childcare costs in the early years, but it's not an issue, why you ask? My earnings have doubled in that time through staying in work. I did 3 days a week for 2 years and then increased to 4 days a week. I've worked my arse off and been in the right place at the right time, to get 2 promotions in that time. Yes it was a gamble financially but it has paid off massively for me and ultimately the DC - we will have a lot more money coming in to give them a better life. And I'm working 4 days a week which is a relief! To do this you do need a supportive DH - is yours? Have you ever sat down and had a proper conversation about sharing the load? Financial planning?
You can do 3 days a week now, do it for a year, try for DC2. When you come back after DC2 you can do 3 days, 4 days, full time if you prefer.
You can always put in more flexible working requests to do compressed hours or whatever. Just check the gov.uk pages on your legal rights. Join a union if you haven't already
There will always be people queuing up to tell you you're doing it wrong, ignore them. You just do the best you can and feel free to change things as you go along.