Ph control in ffof will fluctuate over time and it will exhaust some nute strength as well. One could “ charge “ their mix with ffof ( tea ) ( on bag ) or top dress . You can also scratch in a dry fertilizer to top soil and water in to help carry medium.
i use FFOF in blended mixes also - and find after week 2-3 of flower it needs some recharge. Even tho ffof is prebuffered ( oyster shell ) i always make sure my water and or feed is ph matched to it. Regardless of said buffering….. it makes sense to monitor what i put in over what comes out. Interval top dressing helps.
Ph of FFOF / LW / HF is 6.3 - shoot for 6.5-6.7 ( allowing for ph drift ) then you are set …. No special water necessary just the range of what you put in.
Lastly throw that flush bullshit out window.
When the last weeks of flower are for sure coming , water plant as normal , medium and plant reserves will still carry it to end. Nothing special…..