Evening all
As per title, any recommendations for something fairly lightweight and packable that won't make me feel like a boil in the bag kipper

Bit of a perennial favourite this one.
As far as I can tell, the answer is breathable or packable or waterproof, pick 2...
My own arsenal:-
Gore 'active' roadie jacket - 350g's, totally waterproof but breathabilty average at best.
Columbian Outdry extreme lightweight - 225g's, totally waterproof (I've previously disputed this but it turns out the leaks were from two nicks, fixed with gaffa tape) not very breathable at all
Paramo Quito - 600gs, defo not packable but 100% waterproof and incredibly breathable.

Reg has had some success with lighter stuff and he is a rain god so may be able to offer some further advice. Those gore superlight things have a lot of fans but they are far too fragile (and spendy) for anyone doing 'normal' mtbing

A longterm fan of eVent but now a user of 'outdry'. However, there seems to be many variations. My own experience with the 'Titanium' version is good but can't speak for the rest.

I've also had good results using OMM Kamieika, although in serious weather it does tend to make me feel a little vulnerable when compared to a heavier hard shell ... but that's possibly just me.

I'm a big fan of my OMM kamelika, but after 11 years it's relegated to sitting in the car boot as a spare - got a few holes, missing coating parts etc. I was going to buy another, but picked up some fairly cheap jackets in the Moore large sell off auction, pacamac but cut for cycling type, and that seems to be working pretty well so far. Nothing like as breathable as the OMM though.

I like the OMM as it's stretchy, but tends to be poor a breathing

Paramo best for late autumn winter

Use outdry for summer

Rab pertex shield for day rides and living in the bottom of a pack mostly. Breaths well enough.

Montane event over the head half zip thing that which breaths well and packs down fairly small if the forecast is bad but not constant rain.

Paramo ciclo if it's really sub standard and I won't be taking it off.

Outdry. I kill regular hardshells and DWR treatments so, for all Outdry's "faults", it's the best thing for me

I have packable Galibier Tempest 2, while it's waterproof, it's about as breathable as a bin bag. I also have an Endura MT500 waterproof jacket, and while it's not particularly packable, in so far as it wont fit in your back pocket, it's packable enough that I can strap it to the front harness; which I did on both the WRT and BB200. Of the two, I'd take the Endura every time.

A couple of folks on the second weekend BB200 had what looked like the Albion Zoa Rain Shell, very spendy, but made with Pertex Shield Air. There's also offerings from the new RAB Cinder range...