I'm so sorry to read about Charlie, but I too made the decision last year not to give the Lepto booster, based on the bad press and significant anecdotal evidence of dogs suffering death and illness particularly in relation to Lepto 4. It is a difficult decision to make, damned if you do, damned if you don't, and I'm still not easy about it.
William who is six had the Lepto 2 booster (my Vet won't use Lepto 4) up to 2016 and was tested for exposure to nine strains of Lepto in 2017 which came back as negative.
Has it been confirmed that Charlie had Lepto, I note you say 'suspected'. I too would be interested in your location, is it a Lepto 'hotspot'.
M.