Phosphex bombs on a sarge and rad grenades is a bit too little for "weaponry of the Old Night" in my opinion. First: artificer hardened armour for everyone (to represent armour modified to wihtstand radiation/whatever), second: more weapon choices (special flamers, special hand flamers, grenade launchers a'la grave wardens, some unique guns, mayby some cc). I like the idea of Phosphex boeing thrown into melee but I'd prefere it a'la breaching charge.
Artificer armour wouldn't protect much against Phosphex bombs. In rules terms it requires a reliable invulnerable save (or preternatural constitution like that of the Death Guard's infantry) to keep infantry "relatively" safe from Phosphex. And aren't Destroyers supposed to be walking symbols of the menace represented by Old Night on Terra?
I'm not a HH player, but have been a long time fan of the fluff (before the HH novels were a thing) and tend to model most of my stuff as such, and I'm having a hard time reconciling what this squad is versus what it's supposed to be. It seems that they're supposed to be the keepers of the most destructive and questionable weapons known to man. The stuff that you normally just don't use because it's so darn destructive and literally ruins the battlefield it's used on, pretty much forever. They talk about tactical nukes, ancient radiation and chemical weapons, but the models have fancy grenades and normal bolt pistols. This just doesn't seem right. It feels like they should be effectively Devastators on drugs, but they're instead mildly more annoying Assault Squads.
I picture Devastators with nuclear warheads for their missiles, microwave beams to boil swathes of the battlefield, virus bombs and caustic volatile chemicals that literally wipe out chunks of the battlefield. Molecular disintegrators, acid rains, poisoned gas, etc.
Am I taking it in wrong, or do they just seen to not be what they're supposed to be?
You're not wrong. It's just that there are other thematically-appropriate weapons like Irrad Cleansers and Radiation Furnaces that are already in the 30k game, they're just in the hands of factions other than the Legions that use Destroyers.
and let rad grenades work every round, not just the first round
I think something like that is considered a Relic-level ability by FW. The Barbaran Thurible Relic exclusive to the Death Guard does that already, and that's an "opponent's consent only" item.
You're pretty much spot on. Death Guard Grave Wardens are what Destroyers should be. Big hulking suits of Cataphractii armor to protect them from the horrific weapons they wield, walking around spraying the battlefield with irradiated shrapnel and horrific nanotech or chemical weapons.
Instead, we got overpriced understrength Assault Marines, who can pitch one phosphex bomb that's just going to make it harder for you to effectively assault any unit that you engaged.
I think there is one crucial difference between Grave Wardens and Destroyers. To my knowledge, the Destroyers are, in almost every legion that uses them, essentially a penal unit where legionnaires who have proven themselves too useful to execute outright get "sentenced," where they eventually die a slow death from overexposure to normally-proscribed weaponry. The Grave Wardens are as heavily protected as they are because they're considered a valuable asset, and thanks to their equipment (especially their Cataphractii armour and "Death Cloud" gas projectors) they're quite effective against most targets they can reach while still being very tough.
On the other hand, Destroyers are by and large considered expendable, a unit most commanders wouldn't miss if they died to something as ignominious as a Phosphex bomb mishap, or if they got gunned down by Heavy Bolters they couldn't reach in time (kind of like being stuck in the open against a heavy machine gun with just two pistols in your hands in real life). That at least is a thematically-appropriate reason for why the Destroyers don't currently have the options to get better armour. Remember that the Destroyer Corps is where Crysos Morturg of the Death Guard, a psyker in a legion whose Primarch hated them, got reassigned to, despite eventually proving himself by surviving against all odds in the Battle of Istvaan III.
The major exception to the "Destroyers are expendable" philosophy of use is likely the Dark Angels' Dreadwing force, where they are considered a necessary evil that, if you really need to deploy them, you equip appropriately to let them fight like you want them to win. That's likely why the Dreadwing is described in the HH novels as having access to such goodies like Whirlwind tanks with radiation-spreading warheads that we haven't seen on the tabletop yet.
Still, "Devastators on steroids" would be a good way to deploy Destroyers with no jump packs. Devastator-Destroyers carrying more varied radiation weaponry (like Irrad Cleansers rather than just being restricted to Rad Missiles), access to Chem-munitions-using Heavy Flamers (currently exclusive to the Death Guard) or even Heavy Bolters with Molecular Acid Shells (currently exclusive to the Dark Angels) would be extremely effective.
I'm curious as to how Blood Angels Destroyers will be represented in the fluff, and whether they will receive any unique tabletop rules. Every Blood Angel from Baal hails from a place that was forever ruined by unrestricted NBC warfare, and each one of them almost certainly knows someone close to them who died of radiation-induced cancer or other aftereffect of those weapons. Logically that would mean Blood Angels would be very reluctant to deploy Destroyers, at least against humans who might still be persuaded to serve the Emperor.