Hi,
One potential challenge in this setup is asymmetrical routing. Since the SRXs are not in HA cluster and firewalls are stateful, this may cause issues of non-stateful sessions for return traffic dropped by the firewalls.
Using SRX HA cluster in mixed mode might be a better option. From the QFX, traffic would be routed towards a single next-hop [SRX reth interface] and the load balancing [BGP multipath & per packet load balancing] would be done on the SRX.
If SRX cluster is not possible, then assymetric traffic will have to be avoided. I believe using FBF to implement source-based routing on the QFX routed interface could help achieve that along with advertising preferred routes to each ISP link using BGP MED for instance. This would not be equal load share though.
Cheers,
Ashvin