I think I've finally found the answer. Have I got it right?

[Deleted member 121159]

Two observations on the image you share, which you can read across to your cunning plan.
This guy has their braze-on FD at its fastening positing limit. And the cage is too high (because they can't get down far enough, I assume, not ignorance). There should be a maximum of 1mm between the bottom of the cage and the tip of the large ring teeth. With your band-on, this should not be an issue.
This is a 39t large ring. Your plan is for a 36t. So the curvature of your FD cage (designed for 46t+) will mean it's a long way away (along the chain) from your new small ring (?26t) engagement tooth and therefore changing will not be as 'secure' (it will work).
You may wish to consider a chain catcher.

I see what you mean now. I hope that the difference in curvature will not be a massive issue. After all, a road FD is supposed to handle 50t to 58t front chainrings. (58t here)

 

[Ajax Bay]

Wow! A 58t, "astonishing" . My daughter is only running a 56 (with (sort of) braze-on (no brazing involved)).

 

[Deleted member 121159]

I am well and truly astonished. The parts arrived, installed them, and not only do they work, they look quite good, too.

FD: GRX810
RD: GRX810
Cassette: SLX 11-40
Cranks: Senicx PR3 (24mm axle, fits BB86)
Chainring: Goldix 36-26 (direct mount)
Extra: Spacycles RD extender (Wolftooth Roadlink DM clone)

The only potential problem is that the FD can't get as low as I like. It hits the chainstay, unfortunately. So rather than a 3-4mm gap between the cage and big chainring, there's about 8-9mm gap which isn't ideal. Front shifting is good, though. Chainline wasn't an issue, just needed to adjust the limit screws a bit and it was all fine. I've ordered a 38-28 chainring as well, which will reduce the gap between FD cage and big chainring.

26-40 + 37mm tyres give me a 17.81" gear. I could do 26-42 as well which would be 16.99". To my knowledge, this is the only way to go this low on a BB86 bike. It'd also be welcome news for those who want low gears on their road bikes but don't want to go square taper.

Edit: I should have said the only way if you don't want to pay a silly amount of money for a Sugino Ox chainset (around £500).

 

[grogi.giant]

I am well and truly astonished. The parts arrived, installed them, and not only do they work, they look quite good, too.

FD: GRX810
RD: GRX810
Cassette: SLX 11-40
Cranks: Senicx PR3 (24mm axle, fits BB86)
Chainring: Goldix 36-26 (direct mount)
Extra: Spacycles RD extender (Wolftooth Roadlink DM clone)

So, how do you find the cranks after the first summer?

 

[Deleted member 121159]

So, how do you find the cranks after the first summer?

I had a great summer of cycling. Very happy with the gearing. Chainring was quite worn after 1500 ish miles but was still shifting well. Changed it while cleaning the bike as they only cost about 15 quid each. Cranks are heavy but they just work.

 

[presta]

I can't understand why the OP doesn't have the ability to climb anything with a 25" gear

There's nothing clever about thrashing yourself up a hill in too high a gear, it's inefficient, and that will just tire you sooner rather than later. If you tire sooner, that means you cycle less far, and go home sooner.

 

[Ajax Bay]

I can't understand why the OP doesn't have the ability to climb anything with a 25" gear unless they are on a fully laden tourer.
Is the OP actually climbing trees/walls/sinkholes (delete as appropriate - for extra laughs)?
What is good for you is a reasonable range of gears of which the rider has need (power/hilly terrain/load).
The OP has said they hanker after an "astonishing sub 17" gear" - I guess so as to get up very steep hills, without stepping off.
Which is fine: we are all different, with a spectrum of views and chat preferences.
May you vomit less and live long, @neil_merseyside - what's your shortest gear length?
(Mine's "an astonishing" 25" but I've got a triple, rim brakes (currently, the dark side beckons), a BSA BB, square taper cranks, spd-sl cleats and don't like wearing them down. A friend has a 12" gear: but then his shoes are Size 11.)

This was penned in ironic response to @neil_merseyside saying:

I am really sick to death of all you . . . with the "I can't understand why you don't have the ability to spin the BIG (AKA roadie) gears like I/we/us/(some internet clique) do - so then "you" ask are you climbing trees/walls/sinkholes (delete as appropriate - for your personal (un)funny dig at the less able) "

There's nothing clever about thrashing yourself up a hill in too high a gear, it's inefficient, and that will just tire you sooner rather than later. If you tire sooner, that means you cycle less far, and go home sooner.

Agree: nothing clever.
My next line (you didn't quote) said: "What is good for you is a reasonable range of gears of which the rider has need (power/hilly terrain/load)."
Select your gear range (especially lowest gear) appropriate to the terrain, system weight and strength/fitness.
And resist the temptation to say that 'mine is shorter than yours': just spin to the top self-reflecting on how great that low gear was.
Have you got an astonishing sub-25" gear @presta ?

 

[presta]

Have you got an astonishing sub-25" gear @presta ?

My 11-34 & 22-32-44 transmission gives me 17.5" to 108"

Even 17.5" isn't low enough to get anywhere near an optimum cadence on gradients much above 10%.