
-- what other nutes are you using? I'm not sure that bloom booster has
complete coverage of secondary and micronutes in it,... early symptoms either way, not Mg though, wrong symptoms,.. looks more like Ca to me, with the fine spotting along veins,... that water must be quite soft-- naturally I hope, not made by ion exchange units?
I'd start on a Ca-Mg supplement, and look into those nutes for completeness,... other defc.'s may kick in if they are incomplete, micro's in particular!
I think I see some bug damage too (light colored stippling on upper surfaces), likely thrips or something from the leaf hopper clan,... something sap-sucking, larger in size, and highly mobile! I know well as an OD grower myself :smoking:....they are ubiquitous, so if you stay outside, this is a war, not a battle,... plenty of safe bug killers out there, and I also use SNS209, a organic systemic derived from rosemary oil,...makes the juices taste like crap!
Flushing (in
true soil) is more of an emergency procedure IMO, unless you're a notorious overfeeder,... true it can serve as a shotgun blast reset, but not knowing the underlying cause does you no service at all, only potential set-up for same errors,.. plus, it's a trauma for the roots as well, basically you're waterboarding them-

.. smothering starves them for O2, and depending on how free draining/well "breathing" the soil is, it can cause problems as much as fix them,... personally I use flushing only when the soil pH goes way off for whatever reason, and the usual corrective measures fail to fix things,...