It is not selfish for you to want the DC - it is what is best for them.
You know that he is deeply inadequate as a parent. He may be a 'fun dad' at times and love the kids (as much as he is capable of loving anyone except himself), and the kids may love him, but when it comes to the hard work and mental load of parenting (not to mention paying for stuff), he is just not up to it.
There will be no stability his end: the childcare will be a mixture of his mum, a girlfriend (with a new girlfriend being roped in every couple of years when the old one wises up), and him filling the gaps when he has to.
IMHO your main reason why you can't ask the court for 10 or 12 nights out of 14 (with him having them every other weekend and maybe a regular weekday) is that you need to keep your job - you will need it for decades to come.
You have about 2-3 months until court to consider what is best and what will work with your employer. That may seem ages away, or it may seem like just a few weeks - time feels a bit elastic in situations like this.
Particularly keep notes of whatever you learn about how much his mum does and how few nights he actually has them at his place.