If you really cannot design better primers then to reduce primer dimer I would use a hot start polymerase and use less primer and run a touchdown pcr to try to get only specific binding. For touchdown decide what annealing temperature works for the primers and start cycling about 8c above that temperature and every cycle reduce the annealing temperature by half a degree down to your annealing temperature then run another 30 cycles at the annealing temperature. It seems a lot of cycles but many of the early cycles will only work for one primer and possibly neither primer but it does enrich the specific amplimer for when the pcr starts to work properly

The first option to reduce the primer dimer is to lower the concentration. However, with so much homology it will be difficult.

thank you very much, Paul Rutland and Federico Di Lello