I understand why you made the decision you did for a longed for baby, I would never have done this, but I can see why you did, and hoped the rest would come good, but it hasn't and it is unlikely to.
You have to save yourself now and your baby, if you go ahead with the second child then you do so knowing it is going to be extremely hard on your own, without the resources, without security and two under two is beyond knackering and it will be for another 16 years or more. By signing up for another child you are sabotaging your whole life by a few hormones.
You have your much loved baby, now is the time for seeing clearly the terrible position you are now in, and getting a new job as soon as possible. Split the cleaning and finances and get back to your high earner status asap. Protecting yourself AND your baby so you have options and security.
Your dp has played a blinder, he basically has a baby without the cost, he has a clean house and cooked dinners without lifting a finger or paying anyone else too. Sex on tap, and a 'grateful' wife type at home doing it all and using her savings and paying HIM to be there! Why on earth would he get married? There is no incentive or upside for him now op. I would start letting him know that if you are not married in the next 12 months then you will be reconsidering your future, and Australia looks very appealing.
You have given him everything and paid for the privilege.
Don't compound your mistake by having another baby whatever you do.