A simple solution to the CRCR thing could be that CR racks have slightly lower capacity than regular racks; and/or cost significantly more than the regular ones, so you'd only run them if you needed them. We know that FDev can alter the stats of in-game assets, as they changed the Scarab SRV to have a cargo bay of 4 rather than 2 when they released the Scorpion (which I still say should have been part of Horizons and available to console players) so they could reduce the class 4 CRCR slightly to keep it in line with a reduced capacity for CRCR across the board. Say a stock class 4 CR has a capacity of 16, a class 4 CRCR could be... I don't know, 12? The in-universe explanation would be that the "special materials" used in its construction take up more of the volume/mass so there's not as much room for cargo as there is in a standard rack.
OR
Now that we have caustic sinks, let them absorb the corrosion from inside the cargo racks too, rendering CRCR obsolete and letting you run caustic cargo in regular racks until you run out of sinks. Or have a special module that does the same for cargo racks as the caustic sinks do for the hull, like the experimental weapon stabiliser or the FSD booster.
There's a bunch of different ways to cut it.
The naive solution is some variant on reducing overall capacity like you suggested. That doesn't step away from the (imo) problem that it still leaves hauling corrosives and using CRCRs is a binary on/off switch for corrosive effects[1]. I don't think a flat -50% is unreasonable given you can't just "bulk grab" corrosives under any circumstance and a Class 8 would still offer 128 tonnes, and it's a hard enough drop that means it has significant enough impact on traditional hauling to not want to fit CRCRs as-standard.
Also, if you could remove CRCRs from the game, and replace it as a Cargo Rack experimental modification (which opens the door for needing other Cargo Rack engineering, which is an interesting prospect to consider).
But, I'd rather see CRCRs be a mitigation against corrosive damage, rather than a hard-stop. I like the idea of caustic sinks tanking the damage, but there's implications for larger volumes, as opposed to having the effect organically resolved by the CRCR itself.
To tease out the engineering piece some more... right now, Cargo Racks aren't really "modules" like other modules; they have no integrity or anything like that... so if we make them actually have integrity and a physical location on the ship, that opens the window for a mechanism where Corrosive Cargo causes integrity damage to racks... and maybe MRPs could absorb that damage. Further, engineering could apply effects like "Lightweight"; reducing integrity and increasing capacity, "Armoured" to do the opposite, and many other options.
[1] Where I've talked before that there's a missed opportunity in mechanics around "dangerous hauling" where, among other possible effects, corrosive cargo is one hauling hazard, which could result in better rewards for that flavour of hauling mission. Simply being able to "switch off" the corrosion effect using appropriate racks wouldn't justify any increase in reward there.
UA bombing hasn't been a thing for a long time since the corrosion issue was canonically resolved for good a while back.
A better question is why aren't all racks corrosion resistant now?
Because the issue with UAs (Thargoid Sensors) wasn't their corrosive nature, it was a self-repair mechanism specific to Thargoid Sensors that didn't seem to apply to other types of Thargoid cargo which caused (electrical) disruption to Station services. "Coral Sap" (for example) doing the same thing doesn't make sense, so the corrosion mustn't be caused by the same mechanism.
