Before baby comes I would spend as much time as you can getting your house, finances and other 'life admin' in order. Set up direct debits for all your bills and plan a budget (try to give yourself some extra wriggle room initially if you can).
Have a plan for when you go into Labor and for the end of pregnancy/babies early days. Especially who is going to be at the birth and who will be looking after the other DC. This is likely to be more complicated due to covid restrictions, so the more prepared you can be the better.
There are lots of things, like baby clothes and car seats, which are easy to remember to buy and get ready for a new baby. But having a house full of beautiful baby clothes and a top of the range car seat, is not going to put petrol in the car and food in the fridge/freezer. Stock up on nappies, wipes, toilet rolls and other essentials.
There are a lot of practicalities to consider, and list writing can be really helpful, especially with baby brain. But it's not all about routines and organisation. It is also so important to allow yourself time to process the loss of your relationship and feelings around being a single parent. It sounds crazy but I had to plan times to just have my feelings properly. I would plan to have a bit of time to cry (or more often rage!) otherwise you can get so busy you don't process your emotions properly. That's one of the reasons social support is so important, having somebody to talk to (even if it's just on mumsnet at 3am!) can help you through the worst days, and can give you the opportunity to laugh and have some fun too.
The jobs are never done. Do what you can, and then give yourself permission to enjoy your children and look after yourself a bit. Try to find a balance.