I run the Alko sealed bearings. Would rather have serviceable tapered type but that's what came with the boat.
I'm meticulous with the flushing. In my opinion though, flushing will rinse the saltwater from your brake parts but not from inside the bearing.
When I got the trailer it was 3 years old, never flushed out after dunking, and the bearings never changed.
They seemed fine but I had them pressed out and replaced anyway.
I've done this every 2 years since.
I also have a spare set of hubs c/w bearings that never see water.
I tow 300 mile to my annual hols so these go on at my house before setting off, and get changed over on arrival, then swapped back again for journey home.
Sounds a faff but it's the ultimate in peace of mind.
Trolley Jack, torque wrench, and a bag of one shot nuts - takes 10 min a side - so 20 min added to a 6hr journey.
For local launches I leave the older hubs on.
If all my launches were local I'd service brakes each season minimum.
Spin the hubs each weekend or before each journey.
Change bearings every other season.
If you're launching more than say 15 times a season then change every winter.
If you'd had a trailer wheel go past you at 60mph and looked in your mirror to see your stub axle ploughing the M5 you"d understand!!!
Oh, and I'd agree with 6 go's at pressing new hubs and I'd never re use a one shot nut.