Suggestions for helping my puppy build his tolerance of being frustrated

I have a 6 month old lab x puppy that we adopted at just under 3 months old. His mom came into care pregnant and so he was born in foster care and then neutered and placed up for adoption. He has come a long way since first getting him (was easily aroused, mouthy, extremely confident and reactive). I’ve trained and worked with many dogs however he has definitely tested my ability. He has been in puppy classes / puppy socialization since we brought him home and has come a long way in his impulse control. I am aware he’s in his adolescence stage however this was an issue before but has now increased. He has very little ability to mange his frustration. When we are training and he becomes frustrated we reward a sit or down and calm behaviour. He’s desensitized to walking by dogs and is excellent at wait and leave it, sitting at doors and loose leash walking however he is still easily set off by frustration. A lot of the suggestions online are regarding impulse control which he is quite good at yet it doesn’t seem to be helping with his ability to manage his emotions/frustration. He is still going to training however I’m wondering if anyone has any suggestions of additional things I can start working on to increase his capacity to manage excitement and allow him to learn to settle. He knows place and go to bed as well as kennel. An example of the behaviour is he sits at the door and waits to be released and we continue down the stairs to go outside – then out of nowhere he starts biting ankles and immediately escalates. Asking him to sit he will and I give treats and reward the calm – often waiting for 5 + mins but then the minute we attempt to move again he immediately escalates and jumps to bite arms. Another example is he loves to play and will read other dogs body language but is a bit delayed in his reading – we’ve been helping him by supervising and using high reward treats to pull him out give everybody a break and then let them play again. Often he can engage and disengage however there’s also times where he gets frustrated and escalates to where he is unable to calm down enough to play appropriately. I’m feeling lost as I’m having trouble figuring out why his ability to tolerate things change despite little to no change in the scenario and I’m looking to help him increase his ability to tolerate being frustrated. Thanks in advance