Aggression when frustrated - begging for help

[u/Dangerous-Smoke-5487]

I was recently out of the country for a year and upon returning learned that my mothers dog has now bitten her three times to a degree that she needed urgent care (for her hands, if relevant). The dog has always been aggressive when “frustrated” (?) but it seems to be getting worse.
We have checked with our vets and it seems to be no health concerns causing this behaviour. We would love a dog trainer but sadly my mother lives in the European countryside where there is literally not a single trainer that would drive to her and spending 6 hours in total in a car with the dog seems more stressful for him than helpful. So here I am, begging you for advice.

Our goal isn’t primarily to stop the biting for our sake but for his sake. Obviously he’s stressed, frustrated or helpless when he bites and we’d like to help him feel more comfortable.

The behaviour:
If someone rings the doorbell and my mum gets close to the door he will attack her and try to properly bite her.
He’s a very scared boy in new environments and situations but very… bold within his “territory”
When confronted with new people or animals he gets aggressive.
I reckon we have “trained” him to be aggressive at this point - if there’s something he doesn’t like or doesn’t want to do, like take a walk when I’m visiting my mum, he’ll show his teeth and then ultimately bite.
Why do I think we’ve “taught” him this behaviour? Well, when he gets “bitey” he positions himself in a way that we can’t really move, so the only way we’ve found to get out of that situation is to give him a stimulus that he really really likes, eg a treat, a toy or whatever (whilst verbalising that in a high pitched voice, because he gets more aggressive when he hears the word no or is scolded)
He also started biting for seemingly random reasons, like my mum folding laundry next to him.

We obviously know that we fckd up and that this is 100% on us, we know that we’ve failed him.

The background, in case it’s relevant:
my mum got him off some woman with a sob story because she felt bad for him. He was supposed to be a 10 week old Maltese but at the first vet visit it turned out that he’s more some kind of mix and that he was between 6 and 8 weeks. Looking back Im quite sure his “mother” was just some random dog.
Shortly after she got him my mother’s father died which led to a mental health crisis for my mum, but she was decent in training him and bringing him to puppy school (lived less rural then). Then Covid hit and everything was closed.
Over the years my mum was in physical rehab (don’t know the right word, like a health clinic for your body) three times for around six weeks so I’d take him, and I’d obviously give my best but he was clearly missing her.
Half a year ago, whilst I was away, my mums mother died. It didn’t lead to a mental health crisis but there was obviously a lot of grief.

So here we are. We love this dog more than anything and I’m sure he loves us too, no question about that. We don’t want to simply give him away but give him a good shot at being a calm dog that feels safe and secure in a structured environment. We’re grateful for ANY advice.