Hill chewing machine on a budget

I'd see how you feel with the new brakes before you go spending money on a new bike. Good discs might well be more confidence inspiring than caliper brakes, but there's no reason to mistrust a good caliper. I just rode the Maratona dles Dolomites, which as you'd imagine has some pretty hairy descents, using Campagnolo Athena caliper brakes which worked admirably for the entire ride - and I'm over 80kg.

Good move on the Kool Stop pads for the 105 brakes. I replaced my front brake with a 105 5800 and the stock pads were abusing the rims something terrible. I was digging lumps of aluminium out of them after every ride. Replaced with Kool Stop pads and no issues since.

I went to disk brakes after I found I was descending some narrow hills on my CX stylee with canti's slower than on my MTB because I didn't trust being able to stop safely if traffic appeared the other way

[stufftherules] I know 99% of cyclists manage ok without them and they add weight and they are ugly and take up space but for long descents I quite like in line levers (AKA cross-suicide or touring levers) up on the top of the bars [/stufftherules] but then again I only descend in the drops on wide roads with good visibility on the bends - mainly use the drops to get out of headwind - worth considering?
Ok I'll leave now.

only ridden in Devon a couple of times and it is hilly in a different sort of way, short, sharp and lots of them - mental toughness probably helps as much as the bike and another hopefully not condescending congrats on the losing 5stone

Probably a useless comparison but I can tell you that the cable pull discs on my wife's bike fall well short of the 20 year old cantis on my crosser which in turn seem very agricultural compared to the hydrolic disks on my mate's MTB.

I don't think there's much argument that the very best brakes available are in disk form but good rim brakes are still a lot better than bad disks.

Don't rule out caliper brakes altogether, although disc brakes are better(hydraulic anyway)
I've just upgraded from a giant defy to a cannondale supersix evo, it has 105 rim brakes and they are a lot better than what was on my defy. I have loads more confidence on the dale when descending now due to the stronger brakes.
I to looked at the synapse Al and it is indeed a nice bike, I opted for carbon as I've never had one and then paint job got me to.
I'm sure if you got the synapse you wouldn't be disappointed. But like earlier posts upgrade to hydraulic disc brakes,

A 30 to 28 ratio at 80 rpm will give you nearly 7 mph, a fair bit quicker than walking. Even 60rpm it's 5mph. I like to stay above 70 rpm on long climbs

Stiffness certainly helps when climbing; out of the saddle under heavy torque is when you're most prone to lateral deflection of the BB.

And light bikes make it easier to go uphill. Yes, you can get a bigger effect by losing weight yourself. But you can get a still bigger effect again by losing the weight AND getting a lighter bike.

The number of scolds on here who rush to insist that any kit nicer than theirs is a waste of money is hugely irritating. Buy what you feel. If you love riding it, you'll ride more and get faster.

Of course a new bike (or wheels, or bar tape, or whatever) makes you go faster- it makes you want to ride more and harder!

I actually prefer my alloy bikes with triples up front, for the big climbs. I'd say look for a 50 / 39 / 30 crankset, and an 11-28 or 12-27 cassette.

Nice bike the cannondale. I am 15st, a 50/34 with 11-32 cass will be very useful. Hills will never be easy being so heavy, so expect to be slow and develop pacing or monstrous sustainable 400+ watts whilst climbing

Going downhill as mentioned that is confidence based mainly. There is no necessity to hit high speeds whilst decending. Disc brakes will be better in modulation so feathering braking is easier. When the weather turns wet then disc brakes will be invaluable.

The model cannondale you're looking at has cable disc setup. Hydraulic upgrade when you can afford will give you the best setup

Triple and compact are just two different ways of achieving the same thing. Some prefer one, some the other (see this recent discussion for some opinions).

Confidence when cornering or descending is invariably down to technique. A new bike, nice though it would be (and yes, I'd say the Cannondale probably ticks your boxes), won't fix that by itself. A caliper & pad upgrade can make a significant difference but that might not be the answer you're looking for.

Carbon, alu, steel and Ti, it doesn't really matter. And neither does the weight.