Agree with PP that it would be really hard if I was worried about DS' welfare when with his dad. I would also find it hard for DS to be gone for more than 1 - 2 nights at a time, on a regular basis and I think the younger the child is the more shorter, more frequent visits benefits their well being as three days can be a long time.
Ex and I communicate a little by text sometimes sending a picture, we also do a quick goodnight call. Other than that it's just really good to focus your mind on making the most of being with your DC when you're with them, and line up things to do when apart so you're looking after yourself and hopefully, having some well deserved fun as well
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When you say your ex doesn't put your DC first are you talking about being a bit flaky or things that are more serious?
If you're worried about DC's wellbeing when with their dad then you need to be a bit of a tiger mum and make it very clear what the deal is. Bur if you think that basically your ex will take good care of your DC though, then you probably just need to accept that it's 'good enough' and see the positives in the situation. Your DC get to have a relationship with their Dad and you get time to do all the things you've been wanting to do.
Can't really advise about the manipulation thing other than to say stay calm, be really clear and just stand your ground on the big stuff and don't let him use the small stuff to get under your skin. Imagine you've got an anti bullshit forcefield and it all just bounces off you...the bonus is he will probably find it maddening 