I like Hannah (unpopular opinion!) - she is generally loyal, dedicated, good sense of humour, very clever (reminds me of a partner at my firm, who met her husband at the same firm as a junior). Her portrayal rings true to me - most successful city lawyers dial down their expressiveness around work and struggle to have a life outside. Obvs, she was disloyal to Nathan - when Nathan cheated on her, her anger tipped the balance for her on cheating and then she behaved very badly.
I think that if you rewatch season 1 that it is clear that both she and Nathan are overworked and haven't really done much as a couple - they are plugged into an extended family that is somewhat smothering, are ground down by the responsibility of parenting 3 kids and have lost the romantic part of their marriage. Hannah in particular has spent decades dealing with family drama, dedicating herself to the family firm (under the impression that she will get to do it her way when her mother retires, only for her mum to renege on the offer and decide she can't bear to have Hannah in charge) and having kids ("like a grenade in our marriage") and is resentful of the burden of responsibility. Even without being a partner in a law firm, you only have to spend about 10 seconds on mumsnet to find a thread where resentment = sexless marriage. I think we can all see scenarios in which the marriage would have survived, but yes I think that Kate and the baby is absolutely the nail in the coffin - a fresh start putting the past behind them just isn't available any more and Hannah realises that. Watching Nathan being torn about his guilt about not being involved enough with his child even if he picked hannah would be unpalatable and oppressive. I'm not sure whether he loved Kate or not really, but I think he quite enjoyed how needy she was, along with her being younger.
I didn't like the characters' ending, but I appreciated it probably was for the best. I definitely didn't like the podcast monologue at the end though - it was far too much, overwritten.