@Banhaha, do not consider any of the negative comments from pp's. Thier comments have an undertone of passive rage, I assume from a bad experience of a simillar situation you've posted about. Financially, if it's not ideal to maintain the same cm payments to his ex then simply review and reduce accordingly but fairly. In many families when there are more children the budget has to stretch further resulting in each child receiving less than before. Equally when there is more money available each child benefits from this too.
Lets say you're OH was to become unemployed then he would not be able to make any cm payments to his ex. Unfortunately she'd just have to get on with it and the kids too as that's what happens in the real world. It would be highly unreasonable for him to be expected to maintain the same cm payments without any income. Some mn posters would probably agrue otherwise and you'll get plenty of those silly ones on here.
Bearing this in mind, with the addition of a new baby on the way, it is perfectly acceptable and within reason for him to want to reduce the cm payemts. The addition of a new baby to any family will impact the family budget, so given OP's complicated situation, this is no different. Unfortunately, too bad for his ex if she will now be getting less. It does seem like the ex has suitable options to meet the shorfall such as getting a part-time job or her inheritance.
Similarly, after my OH and I had our baby the family budget was reviewed and his ex had a large cut. She was not happy about it and still is not, however, with time she's adjusted to it. 😋 Like you I also work and contribute significantly to the family money mean while his ex has chosen not to work as apparently she hates working.
Unfortunately some mothers have an ingrained mentality that every penny from thier ex belongs to them and thier biological kids so why should they get a job and work.
Why should the OP's child bear the consequences of an ex who refuses to live a meaningful life beyond cm payments. 🤔