JC wrote:

4/ Now let's suppose my objectives are max F/B as I have a QRM source
opposed to my favourite transmitting direction and SWR 1.5 on a given
frequency range as my transceiver is very SWR sensitive and I can't use an
antenna tuner.
I accept changing wire lengths and spacing but not boom length.
Is there a way to have EZNEC, or another software, doing automatic
iterations until it reaches the best F/B-SWR compromise ?

K6STI's Yagi Optimizer 7.0 does a very nice job of this. You can
choose the weighting of Gain, F/R, SWR or Impedance and optimize over
any choice of frequencies. See my results for a pair of stacked KLM
Yagis on 28 MHz he

http://users.vnet.net/btippett/yagi_optimizer_7_0.htm

Unfortunately K6STI no longer markets his software to hams (due to
software piracy issues). Hopefully you can find someone with a copy
who can optimize your initial results.

73, Bill W4ZV