Hmmm, can't make comment on the acceptable movement issue except to ask what is the movement going to do to the method you use to control the galvanic corrosion issue.

use better bolts. this is the inaction between carbon, Al and Steel. Carbon (insensitive to temperature) is just sitting there. The Al is expanding much more than the Steel, increasing the load in the fastener.

I wonder if the olde Boeing design manual has any insights. If it is that critical to the design, I'm sure smarter people than us have already solved this with clever design approaches.

another day in paradise, or is paradise one day closer ?

S57... Seriously, there is probably not enough information in this issue/question for us to help You.

Secondary structure... what is the Joint configuration? Fasteners?
WHY does the secondary structure have-to-be aluminum-to-graphite? Titanium or even a composite panel [S-glass/epoxy] to carbon-epoxy seems more balanced.
What is the isolation interface between carbon [graphite] and aluminum... corrosion potential is gorilla-in-the-closet!
Real-world thermal design range? Example: -65F to 250F... or....??

I expect that 7xxx alloys are unsuitable much above 225F; 2024-T81 would be unsuitable above ~300F; and only 2219-T6 would be suitable from 325F to [maybe] 375F or 400F [short exposure]. IF temp range is really high... then only special corrosion protective primer-hybrid-blends will survive-long-term above 350F.

Is the problem with high thermal-induced stress [a] axial or [b] biaxial in the aluminum? IF the aluminum part had built-in expansion 'wrinkles'... or keyhole slots [long-thin slots with a large diameter hole at the end/base of each slot]… with fasteners installed thru the edge 'ands' or 'flat-lands' into the composite... MIGHT be a way to relieve/soften stresses in the aluminum fastening along the composite edge. expansion contraction would then be a long-term issue.

Regards, Wil Taylor
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OP - how are you calculating the bolt loads? What are you using for fastener flexibility? What is the length of the joint? Temperature delta?

If you use slotted holes they should be in the aluminum part, not the composite part. But likely not needed.

Split the aluminium panel into multiple panels if you can. These problems are difficult to solve.

Or try something like a wiggle plate, similar to what wktaylor described:
https://www.atsmro.com/wp-content/uploads/2019/09/...

Good point raised earlier regarding integrity of fay surface sealant. If you rely on clearance holes and there is definite thermally induced strains, then the seal will be broken and the aluminium will corrode.

As one might recall about one iconic aircraft, the SR71, its skin expanded so much at speed that it leaked fuel while on the ground: https://theaviationgeekclub.com/former-sr-71-drive...

TTFN (ta ta for now)
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re-reading the OP, slotting the secondary structure to protect the primary structure is ok. How not use simpler large diameter holes in the Al plate and large washers ?

another day in paradise, or is paradise one day closer ?

Design it so it can move.
look at how plate glass in building is mechanically secured to metal structure. I'm looking at a cool spider looking thing in the corner of my office windows.