Yes, of course it is. What I am trying to say was to try not to make it into a "choose them or choose me" situation because.... well, who should he choose? Let down his children who, from the bare minimum you have posted here, have a had a rough time of things? Or let down his wife who has tried so hard to support him and his family? There is no "good" solution for him, as least not as far as I can see.
This situation with your stepchildren won't be forever. You are obviously deparately unhappy and resentful - yet, in this, you probably have a lot more in common with your stepchildren than you imagine.
They won't be grateful to you, not yet, because they probably don't yet have the maturity to realise just what sacrifices you have had to make for them. One day they will realise - but that day may be months or even years hence.
You sound as if you have done lost of things right. Is there any way you could make a bit more physical apace - a loft/garage conversion or a garden office. An extra sitting room for the step-kids or a private space for you to work from? Things like dogs can be good ice-breakers. Could you ask them to they take the dogs out for a walk sometimes to save you the "job", to allow you a bit of time in the house on your own, and make themselves feel useful?
They might well act like sulky brats at times - indeed they may well be exactly that. But sulks are often simply unexpressed anger. Anger at what, I dunno... having their lives disrupted, having to leave home and their friends, anger at being expected to appears somehow grateful for all this...
Things can get better. It won't be easy. But they can.