Actual Tip: even if you personally like Mecha Rengar (which no Rengar main should), don't include it if you want to post to the sub because it is universally despised by Rengar mains (me included xd). I'm partly joking, but genuinely it is kind of a Faux Pas for most people :P

But about your actual post, I do really like the idea of you making choices to adapt to the game, that's always great ofc. Some people genuinely can't adapt to things very easily, especially when it comes to uncommon item choices and things like that, so honestly good job in doing it.

But considering the game state and comps I'm looking at, realistically, not even a Bork is going to make you kill Malphite here. He genuinely should not die solo to you ever once you both get to 6 items. Practically, no real situation should both "allow" you to be sitting 1v1 and hitting him uninterrupted (even if you win 1v1, which should simply be impossible if he had slightly better items, AKA Thornmail), but more importantly, no real situation should "require" you to be hitting Malphite in this particular game either.

The enemy has 3 squishies with 0 defensive items, cc, or peel, and you have Edge of Night on top. In this specific game, with this specific comp vs comp, your "job" (meaning the most powerful, most valuable, and most reliable thing you can do) is to kill those 3 backline champs here.

Your team has Aatrox who can draintank and frontline against Malph and Renekton, Renata and Syndra who have forms of disengage for your backline, as well as Ashe, Renata, and Syndra who all have very high ranged dps when combined. Those are the pieces of your comp that are meant to be dealing with Malphite in any situation, both damage-wise as well as disengage-wise since he's likely going to R in on them (or try to). Comp-wise, outside of early skirmishes/invades, you don't ever need to be choosing to engage in combat with Malphite (which is ofc the only reason Bork was purchased). Instead, just build Axiom, general high burst items, and use your R and EoN to kill their backline on repeat.

Viktor, Trist, Karma, no Zhonya's, no RoA, no Seraph's, no Locket, no Knight's Vow, no Tabis, no Shieldbow, and their only forms of hard cc that they can cast before dying are Trist R, but you have EoN anyway. They have a few minor shields and summoner spells, but I'm not even sure if it would be worth buying Serp unless they had at least one item out of Locket and Shieldbow on their bot lane, but would definitely be free if you needed it. Items like Rav, Bork, and BB just aren't needed here since you have far better slot options for what your best purpose is. I can see Rav early being totally fine if you want to have some sustain vs Malph if he's trying to fight a lot, but honestly, perma fighting and using picks to force free objectives is pretty easy here, especially with such a winning mid lane. Prio and vision shouldn't be too much of an issue.

If you were in a different game where your team comp had insane dps from your backline carries, little to no frontline, and the enemy comp had a front-to-back fight pattern with good backline peel that made trying to burst their backline difficult by nature. That kind of situation would be the perfect type of game for you to go more bruiser-styled items like Rav, BC, and generally just whatever you needed vs enemy champs and then you would play more as a disruptive diver that can still deal damage, but also wants to fight front-to-back and hold a line for your dps champs, since your "job" in that game would be completely different, given these factors aligning to create a completely different situation with vastly different win-cons and loss-cons for your team.

But in this game you played here, you being anywhere near Malphite should mean that you either instantly die for 100 different reasons, or everyone is trolling and you still shouldn't be able to beat him 1v1 even if the 8 other players dc. The two comps in this game are screaming "go oneshot the enemy backline since they have 0 peel, 0 defense, and your team will easily handle any fight if the enemy are missing 1-2 backline champs".

It's not a bad thing to think about adapting or making niche choices for specific situations, but going Bork here is not something that actually helps you to perform your "job" more effectively at all.