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Having gone through the same, I bought a Blackburn frame bag but the volume was less than satisfactory and meant I couldn't get water bottles mounted in the frame. Sent that back and now use the Carrsdice SQR, it does weight the bike weirdly but so easy to clip /unclip and carry into work. The volume is enough for a change of clothes or just my lunch box as it has a roll closure.
So happy to have ditched the backpack in this heat
I have a front rack on the fixed gear which I love, but doesn't look like that's possible with this bike (carbon fork, thru axels).
I had a look at the Tubus Fly with panniers as an option but I don't think I carry enough stuff to warrant it.
One issue is weight. The bike is fairly hefty as it is, I have to lug it up 4 flights of stairs when I get into work and there's also a fair bit of climbing both ways.
That's why initially I thought a frame bag might be a good, lightweight solution to the sweaty back backpack problem.