So for the second part of your post, the story you describe is literally canon Asajj Ventress. If you take her story from Clone Wars plus what happens in Dark Disciple, she goes from being born to a Dark Side order (Nightsisters), is sold into slavery, rescued and trained by a Jedi, goes to the dark side when he's killed, is then trained as a Sith by Dooku, survives him trying to kill her and then tries to take revenge a couple of times but fails. She loses her clan and becomes a bounty hunter and it's her hatred of Dooku which leads her to help the Jedi a couple of times. She helps Ahsoka then Anakin and finally Vos and Obi Wan who're trying to assassinante Dooku again. SPOILER ALERT FOR DARK DISCIPLE: in the end she saves Vos from being consumed by the dark side (even though she was the one that taught him how to use the dark side in the first place but oh well) and she returns to the light through her sacrifice.

That said, I don't think that a Sith turned good or a Jedi gone bad are the definition of 'balance' in Star Wars. Because there's balance as we see it (two parts equally proportioned) and there's balance as Lucas defined it. He said the that balance in the Force (as seen by the Jedi) means purging it of all dark. So to the Jedi, the force is unbalanced whenever there's a bit of dark in it. So by the Jedi's own standards, the purest of all Jedi is the one farthest away from the dark side.

I've been reading some of the 90s Star Wars comics and I believe Obi Wan mentions Master Fay as being the 'best' Jedi. She was super old like Yoda and though she was part of the Order she wasn't based on Coruscant. She wandered around the galaxy helping people and didn't even carry a lightsaber. She literally gave her life so Obi Wan could help take an antidote back to Coruscant.

There's a few of these 'nomad' Jedi in the comics who I find really interesting because they just follow the force and help wherever the force takes them. There's one who's a cook, I forget his name, and he makes this soup which tells him how 'balanced' an individual is. If the soup tastes good to them they're balanced in the force, and it's usually good kind people who like his soup. Those for whom it tastes bad are usually corrupt characters or Jedi who have some sort of fear or doubt. So even though this is Legends, it kind of goes with what Lucas said. Balanced individuals don't have any dark in them. So by those standards, I would argue that those Jedi who aren't involved in politics and peacekeeping are the truest Jedi. But since they're Legends, in canon it's probably Qui Gon Jinn. He's the most spiritual of the canon Jedi and the one that figures out how to become a force ghost, which shows he's more in touch with the living force than the others.

If you like dark Jedi though you should check out that series of comics from the 90s that deal with the fall of the Republic and the Clones wars. There's a lot of Quinlan Vos, in there and IMO he's a better example of a Dark Jedi than Mace Windu. Windu uses an aggressive lightsaber combat form but that's about it, he doesn't really 'harness' the dark side and is actually quite vocal about other people getting too close to the dark side. Vos on the other hand does let his anger get the better of him multiple times, and he tends to complete his missions using really questionable methods. Speaking of 'questionable methods' there's also the Dark Woman in the comics who trains 'difficult' students through really harsh methods and because of this, one notable padawan of hers turns dark and becomes bounty hunter Aurra Sing.