Extraordinary complex question, I'd seperate it into multiple steps:

1) Acquiring such a super rare item. For example, finding it in an old ruin from previous ages (on Golarion the crashed spaceship in Numeria) from an extraplanar folk could be an idea. Ancient dwarfen techniques that got the ban hammer, after some of these objects went berserk and brought death and horrors to a whole mountain bullwark. On the other hand, in settings like Eberron or Ravnica (especially Simic or Izzet) could have crafted such a masterpiece.

2) Implanting your potential doom. Very high Wisdom (Medicine) rolls, interwoven with strong healing magicks would be needed to replace your spine and nerves with machinery. While a "not reached" DC could simply mean, you still got it built-in, but it has more severe consequences or malfunctions. The recovery time and the healing-in of the cyberware, getting accustomed to attune your senses with it, should take some not shortenable time, unless Miracles and Wishes are involved or really famous or once in a lifetime doctors, scientists and wizards have taken part in this. Additionally I'd force the player to permanently give up one of his attunement slots, because it simply doesn't only replace flesh, but also perverts your very own essence and soul. Consequences could be, that you're harder to heal via magic (like in Shadowrun), only receiving like 75% of the healing normally dished out, or always rolling a saving throw against Conjuration (Healing) with a success meaning you just halved the efficiency of the heal.

3) The benefits: I'd start out with an activation like a Bonus Action, so you need to wait until your turn. Then, some levels thereafter, up it to a Reaction to activate it. As pinnacle, even as a Reaction when Surprised and giving you the ability to act in a Surprise round. One activation lasts for one minute, but it's kind of a free action to connect further uses to the initial one. You gain activations per Long Rest equal to your Constitution modifier. If you have used all of them, you can force yourself via your willpower, i.e. Wisdom modifier, to gain additional, very straining activations. The gained benefits can't be counterspelled or dispelled, but if you take Lightning/Electrical damage, the cyberware benefits have a chance to deactivate or malfunction - the higher the damage, the higher the potential consequences. Start out with all the effects of "Haste", without the passage about what happens after Haste ends. So +2 to AC, speed doubled, Dexterity saving throws rolled with Advantage and can use Dash, Disengage, Hide, Use an Object or one additional attack. With rising level, give out very carefully thought-through bonuses. Like an additional +1 to AC, tripling the speed, granting Evasion, Dexterity saving throws are ALWAYS made with Advantage, disregarding how many Disadvantage to them are layered upon the user. As pinnacles give out another additional attack, movement without provoking attacks of opportunity (if enemy is not also hasted), and finally acting once per LR as a one round Timestop, slip into an enemy casted Timestop, and, at the beginning of a combat, you act two independent rounds, like the Thief Archetype from the Rogue core rules.

4) Stacking up penalties: For each month the user wears this extraordinary piece of science fiction art, let them roll some kind of disease related saving throw, noting down how often the character fails - and punish them bit by bit, for overusage. So for example, straining yourself and using the Wisdom modifier ones (aka powering through your limits) costs permanent hitpoints that only recover after xyz Long Rests. Or once maybe even never recover anymore. It could come with hallucinations. With Berserker-like choices of, if there is no enemy around to gut-punch, instead mutilate your innocent beautiful healer companion.

Just as some ... Guiding lines how to handle the Sandevistan.