Originally, I read OP post and agreed NOT BU. The more I read, the more I changed my mind - not so much about the initial question, but more about what a seemingly awful and negative attitude OP seems to have towards DSC.
You can tell yourself you're a decent, fair, good SM but the way you talk about them etc is really sad. And they may seem OK and happy but children are sensitive and step children, even more so. They will definitely be getting your vibe and picking up on the resentment that, you can deny all you want, is clearly there.
Your DH was their dad before he was you DH. They didn't choose for him to leave them. They didn't choose for him to remarry. They didn't choose for him to have new children who he lives with and hasn't left. This is their reality. They've had zero say in it.
And even though you've stated you are not their second mum, and from your numerous, lengthy posts, it seems clear you have no interest in that role - it doesn't mean that DSC don't see you that way, especially the younger one.
Also, you are either deluded or purposefully minimising the reality of the DSC's situation! You really think his older DC get more 'quality time' than your DC?!?
He left his older children.
He no longer lives with his older children
He lives with YOU.
He lives with YOUR children.
"Yeah okay he might have tea with the youngest more nights of the week after work and the business of getting everyone ready for bed, fed, bathed and whatever." this - right here - THIS IS QUALITY TIME. MOST CHILDREN WOULD CHOOSE THIS OVER EVERYTHING ELSE - to live day to day with their parent(s)
" But actually end of the week, focused, quality time... They 100% get more of that" THEY'D SWAP IT IN A HEARTBEAT TO HAVE THEIR DAD LIVE WITH THEM.
Whether or not you want/agree to spend regular quality time alone with your DH and DSC seems irrelevant by the end of this thread. The more pressing matter may be for you to speak with a completely neutral 3rd party (professional, maybe) to work through your denial of how you view and think about your DSC.
The tit for tat stance seems churlish and totally immature. The lack of acknowledgement of the actual set up and what constitutes quality time is what's frustrating a lot of people who've replied to you and, comes across at totally inconsiderate of the DSC on your part.
It's always 'funny' (not haha) to see AIBU posts from people where it's clear very early on that the OP has zero interest in what others think as they already made up their minds and they come to MN with the sole aim of their unreasonableness being justified.
The short reply - YABVVU and I feel so sorry for all the children involved.