I am just grateful he’s not on a downward spiral of drugs or alcohol and that he hasn't tried to hurt himself-
Yes that is all to the good.
instead he displays this arrogant irresponsible attitude which I can’t seem to change.
Meh, he is 19, it is OK for him to be arrogant and irresponsible. He is leading his own life reasonably successfully, the therapy seems to be helping.
The impact then on his half siblings are so detrimental.
Why would there be a detrimental impact on his half siblings? Maybe you are expecting him to be too engaged in family life, when he is at an age when most young people are building up their own independent lives and separating from their family. If he never got on with his stepfather then maybe you can't expect him to want much to do with his step-siblings either.
It’s all so emotionally exhausting.
Let it go. A lot of it's unnecessary now. Concentrate on letting him live his own life and building the best relationship with him that you can.
since he turned 18 I was lucky if he responded to any of messages of why he was late coming home at night or answered any of my numerous calls.
At 19 he can come home at whatever hour he likes so long as he doesn't wake the house up or bring home dodgy company. If he lives at home there is no reason for him to be answering calls from you. And if he's not living at home then he'll call you when he needs you.
I don’t know what to suggest of what we can do or where we can go to spend some quality time together without feeling pressured or stressed.
Absolutely do keep on making time for him - just pleasant relaxed time together. That will help to heal your relationship. Don't feel that you must have serious conversations all the time. Cinema, walking the dog, coffee or pizza, anything really that's just the two of you. What does he like doing? The time together may have to be limited but while it lasts it is up to you to not let other pressures intrude.