All queries invoked from a composite query share the same 5B instruction limit. So it is not possible to do more work by splitting a query into multiple queries.
As I mentioned in another thread, the main blocker is that the canisters need to somehow pay for the work done in queries. @stefan-kaestle explains the problem of charging and potential solutions here: Community Consideration: Explore Query Charging
I think so far the discussion was about either enable query charging for all canister or none. Your suggestion of some opt-in mechanism sounds promising: a canister would indicate in canister settings that it is willing to pay for queries, then the system could increase the instruction limit for such canisters and start charging them.
Would you mind posting this suggestion in the query charging discussion?
How many instructions would you need per query?