Difficulties training fear reactive border collie

So as the title says, I've got a 6 year old pure-bred border collie (Poppy) who is fear reactive. She's the sweetest girl at home, and is more than willing to curl up on the couch with the family when asked. However, as soon as she's outside, be it in the garden or on a walk, she switches. In the garden it's manageable, we've got cues to get her to come in, but she fixates and nothing will break her attention. The neighbours mow their lawn and she goes ballistic, next door lets their dog out and she's at the fence freaking out.

She's a lockdown puppy, as I'm sure you can tell from her age, which I think has contributed to the issues. Our older girl (Willow) was put to sleep last January at 14 and now we've just got Poppy on her own. We couldn't walk them together when our old lady was alive because the manners on lead were non-existant, think one dog walking nicely at heel and unable to walk very fast or far and the other trying to drag you along.

On walks, she fixates. We've found no way to break the fixation, barring dragging her away from the trigger until she's able to listen again. She pulls constantly, and with no treat or toy drive on walks, there's no way to train it out of her force free that I've found. She's in a martingale collar because she had a habit of slipping them when she saw something she wanted to get at or ran out of lead and we'd refuse to be dragged. Most recently, she saw a cat on a walk, this cat did not have any fear of dogs (frustratingly) so I made her wait until it had stepped off the path before we kept walking. She's seen it under the car and locked on, ears up, tail straight and refusing to walk forwards unless dragged. I felt awful, but I practically had to hold her up by her collar so that she couldn't get at this cat if it decided to step out from under the car. Even being dragged forwards like this, she's trying to turn to look and watch it.

I don't think she'll ever be a dog that can be trusted off lead, which is fine, she can get enrichment in other ways, but she needs to learn to walk on lead for our sake. I've stepped on her more than once because she doesn't have a personal space boundary and gets underfoot. I don't like having to constantly pull her back into heel, or stop her bolting across a road. She won't even pee or poop outside of the garden because of how keyed up she is all the time.

Any advice would be appreciated.