Sorry. Saw this after I already posted...
I can't speak for Fastjack or anyone else, but I personally take no offense, and am totally fine with you getting the last word! I think everyone's stances are pretty clear and we managed to not light anything on fire.
I suppose the two types of augmentation could instead both use Body Index instead of Essence, but it'd just end up nerfing mundanes by lowering their pool of augmentation resources. Well, for most characters anyway... admittedly going all-in on Body is rewarded in 6e and the potential to go beyond 6 raises interesting possibilities. A problem would be in keeping awakened characters from getting the best of both worlds, but the "MAG drops every time your ESS drops" rule could probably just be changed to a "MAG drops every time your Body Index drops" rule.
I think SR6 took a good 'Gordian Knot' approach. While you could totally re-invent the mechanics of a limited pool, the ultimate 'problem' with essence is that it presupposed some sort of essential baseline humanity that altering 'perverted.' So it was ultimately a form of 'purity language' common to prejudicial thinking, regardless of if that was intentional.
Most mechanical aspects of essence were fine (reduced social limit wasn't, but that is such a non-mechanic anyway you can remove it and change nothing of the game balance). Even the fact that augmentations allowed some mental drawbacks as an option was fine, because it being optional moved their existence away from being a an inevitable ramification of messing with the 'pure human' baseline, and was more about how someone who already was maybe succeptable to negative psychological states may be worsened greatly by suddenly being able to crush others with their bare hands or read emotions or affect emotions.
So if you just say that essence merely is a mechanistic bodily function as morally charged on a biological level (ignoring the sociological ones) of getting a blood transfusion, which 6e sorta did, you can more evenhandedly explore all of the more mixed ramifications of 'ware (You have things like body freedom, and the ability to define your physical form free from what Biology limited you too to explore your identity, vs things like pressure to become a tool instead of a person, and the unequal access to 'ware which now is moral neutral being a much larger condemnation of society) while still keeping the ability to talk about people who lose themselves in it. It also helps move magic away from the 'golden boy' space it is by painting a MUCH darker picture of people like Man of Many Names (Who pretty much is openly a biggot, saying things like mages deserve more pay and screaming at people about 'ware, but the framing is very kind to his point of view), and avoids playing into pro-neurotypical tropes equating people who struggle to read others or be read as 'less human' if you REALLY gotta keep the social penalties in, because now the coding of 'ware being MUCH less negative makes the (unintentional) parallels to autism less grody.
Posthuman stuff
I am always personally skeptical of any mechanic that tries to break the essence limit at the expense of another budgeted bit of power. The interesting thing about essence isn't even a power balance thing so much about forcing you to make choices about what you value (With some builds being incompatiable with some very useful tools). This specifically favors limb builds a LOT, and is very unkind to non-physicals in mundane roles.
That said its not really a terrible ruleset, it just doesn't fit my own personal 'vision.' If the idea of 'limb builds not being able to fit synaptic feels like a bad thing for the game rather than a good thing (and I can see it) and you want to increase parity with adepts (This would require adept buffs of some sort, obviously, but they need it anyway), it is a good system.
D20 'ware
This seems interesting, but extremely overtuned. In SR, you lose 1 social limit (a far more mild penalty) per 3 essence of 'ware, while here you lose 1 social MODIFIER per full essence AND the essence costs are trippled, meaning for every 1 ess in SR's cost system you spend, you take 9 times as many penalties. A single cyberlimb goes from being 1 essence and thus -.33 social limit to 3 essence and thus minus 3 to social rolls, or a total effective charisma penalty of -6. Two cyberlimbs goes to giving you an effective negative charisma modifier of -6, which would be the equivalent of a hypothetical -2 charisma if you started at 10. Getting past that would require you to spend 3 whole levels dumping skill points into all your social skills to merely hit 0, which kinda just means you can't use social skills. Furthermore, while the MAX base charisma is 18, 18's are extremely expensive in point buy, and even not dumping charisma you just removed around 44% of the average 'ware budget. A baseline human in this system literally can't equip 'wired 3, you would need at least 16 charisma to get it.
This is an extreme nerf, and it makes social 'ware users effectively impossible because a -1 in D20 is way bigger than a -1 in SR (If you lost a full charisma dice per-ware point in SR, for example, you still could make a .01 ess face, the same is not true in D20). I think you need to tone it way down, and reconsider if it shouldn't be con that affects your max (As SR goes out of its way to make social rolls easy even if you dumped cha for a reason, its interesting to have every PC able to talk in a story).
That said, I think toned down it would be a great add for a system where the augmentations were 'mental hacks.' EP2e has a concept where characters give themselves mental hacks that are essentially a form of autism, literally optimizing in universe towards certain things that matter to them. While it mostly exists as a clever form of justifying some optimization choices (without demonizing them, EP is even in more repressive areas generally pro-neurodiversity and thus wouldn't judge someone who is anti-social but loves numbers or is super empathetic but also spatially challenged), it may be a neat system to represent doing that mid game (with a greatly reduced penalty).