This is my method & I use a fermzilla
I ferment without any pressure initially, and purge a liquid filled (saniclean) receiving keg (cut gas diptubes so I can release all sanitizer) with the co2 from fermentation. I wait until after a few hours of active fermentation before I hook up the receiving keg to ensure air has been purged from the fermzilla prior to the keg purge.
I then dry hop some time shortly after after high krausen, but with at least 25% attenuation yet to be achieved. Basically as soon as fermentation starts to slow a bit.
I hook up my tank at 2 psi (this ensures positive pressure when I open the gas post for dry hop) to the liquid post (remember to always purge all gas lines line of air prior to connecting to any gas posts during all points of the process) . Then I unscrew the gas post & quickly add 5g of ascorbic acid along with the dry hops (via a homemade funnel built from a pet bottle). Then I screw the gas post back on & hit the lid with 25 psi, purge once and hit the lid again with 25 psi. Spunding valve is then set to 25 psi for remaining fermentation. After dry hop I also remove temperature control to encourage the yeast to free rise and quickly complete fermentation while scavenging oxygen. If my gravity reading is too low at dry hop time and I may have missed the window, then I add 10g of dextrose along with the ascorbic acid.
After adding the dry hops I wait a few hours to allow oxygen to be scavenged, then I shake the **** out of the fermenter to kick the hops back into suspension. I do this a few morn times over the next 12 hours then I allow the yeast to settle out. After 24 hours total I do a closed transfer to the keg & set spunding valve on keg at 30 psi to allow fermentation to finish in the keg. My floating dip tube in the fermzilla is a flotit with dfi screen to ensure hops aren't sucked up at transfer. My receiving keg has a clear beer draught system dip tube without a screen to ensure no clogging during transfer.
The final serving vessel has now had just about all oxygen scavenged prior to the eventual cold crash after target gravity is reached. It is also free of particlate hop matter & mostly carbonated with pure fermentation sourced CO2 as opposed to bottled CO2 (which is only 99.95% pure)
I prefer free balling the hops vs adding a baggy
I also use epdm rubber o rings on my kegs which are much less O2 permeable than silicon.
I also only hook up gas & liquid lines for serving. When not serving they are disconnected. All of my lines are eva barrier that are very resistant to O2 permeation
If tje keg is undercabonated I may leave the gas attached for a few days at desired psi level to allow carbonation to finish & then disconnect gas once achieved
