Well perhaps unsurpringly the Rocoren unfortunately was not behaving nicely when it arrived.
Wouldn't even negotiate 45w fast charging with my phone when first plugged in or the car was switched on (let alone the 140W it's meant to be able to do...), you'd have to unplug the cables from it completely, wait 20 seconds or so and then plug back in for fast charge to suddenly work. Needless to say as this was being quirky from the word go, it didn't fill me with hope it'd perform as expected, or last, so I've requested a return on that and have ordered something slightly lower spec from Baseus, who I've seen thumbs upped for good chargers before. I was tempted by a Ugreen as they're usually solid, but I saw Ugreens higher spec model seemed to be suffering from issues with overheating/struggling to deliver its rated peak power output so skipped them.
The Baseus model is still meant to be able to do 100w via PD/USB-C/PPS and is meant to support 15V/3A as needed for the Steamdeck. Slightly more reassuringly it makes a deal about supporting numerous laptops in the listing and a decent number of reviews mention success with laptops; it seems maybe it uses some of the same chips as the likes of the Rocoren, but Baseus have limited it a bit more by restricting it to max 100w when only the USB C is plugged in, or 65W on the USB C and 30W on the USB A when both are plugged in, and they've not tried to cram in a 3rd connector. Many of the reviews mentioned it stays fairly cool even when doing a high wattage charge, and was more reliable than other similar/chinese chargers they had tried, so will see how this one goes when it arrives. 100W max is also much less likely to cause issues with the power socket (12V x 15A fuse usually, but some cars its 10A).
Baseus do also do a higher wattage charger, but the reviews looked more mixed on that, plus it was twice the price, and I really don't need more than 100W USB-C right now.
Edit: Initial signs with the Baseus are MUCH more promising than they were with the Rocoren, despite the more conservative headline figure of 100W v 140W, as soon as I start my car and power kicks in, it negotiates super fast charging with my Samsung phone (which is up to 45W depending on how low the battery is). It has a little display to show power draw so I may take my work laptop out, use the battery a bit, and then see if it's able to deliver approximately 100W as promised. Assuming both these work, I see no reason the Steam Deck won't, as the unit is also meant to support the necessary 15V/3A as the Steam Deck desires, and, if, unlike the higher wattage units, it run's fairly cool, it should hopefully continue to work for quite some time, as not suffering heat related fatigue in the same way!