Back with an 8 month old border terrier here - the bitey madness eased gradually, then we had endless zoomies, and now at adolescence he just constantly attention seeks.
He hasn't really been left alone due to lockdown and me WFH - we can go about our normal routines with no problem, he waits patiently for us to return from a shower, doing something in the garage, etc, but actually leaving him 100% alone - while we're not anywhere on the property, even just outside - has been a maximum of about 15 minutes. I've done that repeatedly though, leaving him with a kong, so he's accustomed to it.
He's never been distressed, never destroyed anything or gone mad at seeing me return, but he is always by the door. If I go to the shops and leave him with DH though, he apparently fusses and paces and goes back and forth from back door to back gate until I'm home again. He doesn't cry, he's just super restless. So I don't really know where to go with that, as somebody is still with him while he's fussing about my absence - he doesn't do it when DH is out. It's possible he's over bonded with me as I'm always here.
Don't you find that a kong or a licki-mat only lasts about 5-10 minutes? I'm still at "beginner" stage with his kong so I don't make it too hard for him, he's got it sussed - but I'm wary of making it harder too soon as he might just lose interest.
Our issues at present are mainly with extending the time that he can be left, we need to do more on that. But he's also developed some hysteria in the car - he screeches blue murder whenever we slow down or stop, presumably in anticipation of one of us getting out - even though one of us always stays behind with him, we haven't left him alone in the car yet.
His recall is great as long as there are no other dogs around - I daren't let him off completely as he tends to race up to other dogs and jump in their face, which he's only going to get away with as a puppy for so long before other dogs start resenting it. So I'm caught in the cycle of wanting him to go off lead and learn for himself how to calm down his approach, and not wanting to be THAT annoying owner who can't get their dog back. Or worse, he rushes up to the wrong dog and gets hurt. You can just say his name and he'll come tearing back, but if he's spotted another dog he'll be gone and that's that - no chance. We tried a 10m line but if he smells a dog before we've seen it he's already out of reach, so he's mainly on a 5m flexi. The dog walker let's him off with all the other dogs (he can call back the others no problem and mine will just follow them) - he goes at least once a week for the socialisation.
So there's that. Oh, and he still pees indoors when the ground is wet or it's raining. We've tried EVERYTHING, taken guidance from a trainer and tried very method, but now it's become a "big deal" and he's picked up on our tension, so he's even less likely to go outside when it's raining. He's steadily wrecking my carpets. 
He attention seeks all day every day, whining and pacing while I'm trying to work. I'm on a border terrier Facebook group and someone sent a link to a blog about the "attention diet" - we removed all his favourite noisy toys (I feel so horrible, he loves his toys) and actively ignored all the attention seeking behaviour.
It worked in less than an hour, and after a week he's been much better and far less needy. You have to harden your heart a little to be able to steadfastly not speak to him, make eye contact or laugh when he's waggling and slapping his paws around and being adorable. He's killing me.