If you think that Manectric has the worst level-up movepool out of Electric types, I invite you to look at Tynamo.
Moving on, tbh, Manectric gets about everything it needs to be a decent special attacker through the natural progression in the game. Discharge is highly spamable in Reborn, even getting a boost on certain fields like the Factory field, as well as being excellent for otherwise troubling Double battles by potentially doing tons of damage and paralyzing two Pokemon. It also gets one of the best moves it could ask for by the time you obtain it via TM57: Charge Beam. The potential to hit with decent STAB damage while setting up is nothing to be slept on and it allows Manectric to actually sweep many otherwise challenging fights as a revenge killer/cleaner. You almost certainly want to soft-reset until you get the necessary IVs for HP Ice, however, as it will be invaluable later on. Hidden Power is also obtained naturally after Noel, so before then, you just need to make sure you have some support for Manectric to help him with Ground and Grass types (another Grass type works pretty well here). Flamethrower does come a bit late, but it is still a solid enough move. Thunder is another option for pure power, esspecially if you want to combine it with Ludicolo to handle each others problems and have both benefit from the Rain that Ludicolo can set up via Rain Dance (Lotad learns via level-up at 27, which is would need to still be Lotad at the level anyways to grab either Giga Drain or Energy Ball).
It is also worth mentioning that Electrike actually learns T-Wave as well- and while you point out Magnemite learns it, at the level Magnemite is obtainable, you will have to utilize the move relearner or the daycare to actually have it on Magnemite, since it is catchable in the low 30s in Reborn and learns it at level 13 as of ORAS. And to breed it onto Magnemite, you obviously must have a Ditto since Magnemite is genderless, so if you want to save your heart scales, T-Wave Magnemite has worse availability than T-Wave Manectric, since Manectric can breed with anything in the field egg group.
I will admit that Magnezone has much nicer general utility in its abilities, but Lightning Rod is still excellent against the AI, since you can easily trick them into using any Electric type move and proceeding to destroy their lives I'm looking at you, Shade. Magnezone does also have a significant advantage in base special attack, although this advantage is mitigated if Manectric polishes off a Pokemon with Charge Beam and gets the boost, leaving it stronger than Magnezone. While Magnezone does obviously have access to Charge Beam as well, Manectric does it better with its superior speed.
Plus, while Magnezone does have an absolute ton of resistances- we're talking 11 resistances and 1 immunity here- it's kinda rendered moot when you consider that every significant fight after the Magnezone line is obtained (with the exclusion of the most recent PULSE) has had coverage to nail Magnezone on the few weaknesses it has. Most commonly through Focus Blast, but sometimes through the likes of strong ground types, like Nidoking, Gliscor, or Garchomp. It's certainly noteworthy that while Magnezone does lose to these mons due to being outsped and KOed by SE coverage, Manectric can actually beat them through Charge Beam+HP Ice, coupled with its high speed. For example, on Ciel, you can whittle down her other Pokemon with Charge Beam, slowly boost up while he insists on healing, and them OHKO Gliscor when she finally sends it in because you outspeed. Manectric is also just fast enough to outspeed Garchomp, which is a significant benchmark to pass.
All in all, I find you claims that Manectric is a terrible Special Attacker in Reborn are rather baseless. Yes, it does require a bit of assistance from Charge Beam and HP Ice, but I honestly never once found that I needed any more than Electric STAB and HP Ice for the entirety of Reborn up to the current release. Speed is the main determinant in the differences in these Pokemon, and while Manectric does have its flaws, it is a valuable and powerful Pokemon that offers many assets. And while I just spent a decent chunk of time defending Manectric and I love to use it, I still believe that Ampharos is actually the best Electric type in Reborn since it has the ability to be either a slower, bulker mon, similar to Magnezone, with nice support movesl like T-Wave and Cotton Guard, or to be faster and stronger, similarly to Manectric using Agility and Charge Beam. Plus, it has a very nice level-up movepool and its Mega Evolution only highlights these strengths.
Also, I gotta say, Mega Ampharos beats Mega Manectric almost 100% of the time because STAB Dragon Pulse can one shot it, while Manectric's HP Ice only does 40-50%, assuming it's not a particularly bulky Ampharos.