Discussions
We have a Dawes Discovery (Pretty sure it’s 21/17 as well) and use Tubus Cargo rack.
While I think it would be tight to get the seat all the way down as is; the poles that attach to the top of the seatstays have quite a lot of adjustment to slide the rack towards the rear.
We also have a couple of bits of metal attached to the ‘legs’ at the bottom of the rack, where they would normally bolt to the bike to move the rack back ~15mm to clear our disk-brake calliper / Bob trailer hitch.
Perhaps you could find a rack that enables some adjustment where it attaches to the top seatstay like ours, and fabricate a couple of bits of metal to put on the legs to move the rack more to the rear?
I think our rack has the official Tubus extension piece, (https://www.sjscycles.co.uk/carriers-racks/tubus-rear-rack-extension/?geoc=ID ) but you might be better with a custom piece to allow you to shift the rack further back as required.
Hopefully you can see what I mean from the photos, we seem to have parked up at a neon-disco tonight!!!