Harpie deck is pretty cheap, they have staple lighting vortex as a plus you could play 2 off or even 3 if you are desperate. They don't need any other extra deck to reach legend. They could play the barrier you get from Rare card as discard folder. They world with a bunch of skill. The lightdark mirror is useful there. You only need to open 3 mini box max. faster if you only focus on getting 3 channeller the rest could be at 2. Even 2 chaneller are enough for consistency.
In the process you also get Harpie Cyber clash, arguably the best level 8 synchro now. Daigusto Ermerald, a good staple rank 4, And Lighting Vortex taphat every meta deck play 1.
Harpies problem is that the key cards to make it competitive are in other boxes. Namely Lance and Swallows nest. Without this 2 the almost have no chance going 2nd with a single karma cut set.
Then you'll want to get at least 4 of any of this so that still 2 main boxes worth of gems. I Agree that harpie is strong, the problem is that the initial normal summon is too important that competitive build are around protecting it. Also F2P build will have no disruption except for slash going 1st.
Look at what you are asking for, now you don't just want any easy to build deck for f2p you want a top competitive whale deck that has all weaknesses covered, there's no f2p like that, a f2p build is gonna be interior to a 100% optimized one, always.
There's disruption besides slash you can run the spiritual art trap or e-con, you are just asking for too much for only 3 miniboxes
The moment any deck key okay got interior they also couldn't do anything. If you don't play many nest or lance, you need to play discard traps version with 3 hysteric sign to interrupt your opponent and plus off it.