There's quite a few questions there Peter, I'll try and separate them out/
I could go Spyre road calipers and change the levers to Tiagra flat lever?
Peter
First one, yes. The Spyre and Spyke are the same brakes with different cable actuation. Shimano changed the cable pull on all their road groups a few years ago and ideally you'd need to find out if the Spyre is optimised for the original or newer and buy the correct levers, though the difference will be in feel rather than braking ability.
If I go the hydraulic route, most of the levers on Hydraulic brakes are very short and look too short to use with a rohloff shifter.
There's a few longer levers, I have two types. Shimano Deore T6000 (Deore comes in two flavours, M for MTB and T for Trekking) which is identical to the M6000 other than the longer levers. I don't think these have ever been commonly available in the UK, I bought from Germany, though that's no longer as easy. I wanted a long lever so it better matched the V brake lever on the other side. Shimano's lower groups. primarily aimed at hybrid type bikes, also have longer levers. I have a pair of these (BL something, the lettering has worn off) they came on a Kona hybrid and will be going back on another build shortly. In all respects, I haven't noticed any difference between these and the Deore.
Levers and Rohloff shifter - Not on my Mercury, but on another bike I have the M6000, there's no conflict. I've been out this evening on that bike, having read your post, I paid some attention to it - my hand it partly resting on the shifter as I brake, that might have been an issue with the triangular one, it isn't with the current design.
EDIT - This is with the twistshift specific Ergon GP grips. I've recently notices some people using full length grips with a Rohloff shifter, I'm not sure of the thinking behind that.
OK, that's the factual stuff, would you like some opinion
I have had three cable disc brakes, Avid BB7, TRP Spyke, Paul Components Klamper. They all work fine, they all stop you as you'd expect.
The Spyre is the only one where both pads are moved by the lever. The advantages, if any, are too small for me to notice. The disadvantage is that the mechanism had to be made small enough to fit between the rotor and spokes and as a consequence the components suffer more from wear and corrosion. After two winters of faultless performance mine started playing up, the adjustment that is fiddley to start with wouldn't stay set. I tried various remedies, before giving them a complete strip down and rebuild - that wasn't an experience I have any intention of repeating.
BB7's are a bit agricultural, to look at and in use, but they work fine. They do feel a bit rough after a while, but are easy to strip, clean and grease. I'm not keen on the pad retention, where they slot in without any retaining screw/pin. I've never had, or heard of, a problem, but I like the reassurance of a physical retainer.
The Klamper is a thing of beauty, as it should be considering the price, works like a BB7, just better. Smoother, nicer feel at the lever, easy to adjust, looks easy to strip (I haven't had to do so yet) There's a vid somewhere explaining why they chose to just have one moving pad.
All three recommend adjusting for wear by moving the pads in rather than shortening the cable. If you do just take up the slack with the cable, the TRP will be less effected than the other two.
Hydraulics - I've only used the three models already mentioned. I chose Shimano because I prefer the idea of mineral oil rather than DOT. I swapped the Spyke on the rear of the Merc for the Deore T. I find the difference between cable and hydraulic discs to be greater than the difference between cable disc and rim brakes (Of any flavour) It isn't that I've ever had a brake that wouldn't stop me, it's just the ease of doing so and the control that gives you. Plus, by their nature they're self adjusting, and when they do need maintenance it's simple (Though something else to learn) I know, - can't be fixed at the side of the road, but then neither can a lot of my bike. I wouldn't choose them for an exotic expedition, but otherwise I'd need a convincing reason not to use them (Like on my folder)
Last point - the way I use my Mercury, I chose the 853 V brake fork, I've never doubted that was the right choice. When I bought a bigger tyred tougher bike, the right choice was a disc fork.
