Homebrew Feat: “Whirlwind Sentinel” –critiques and suggestions please

The DM and I have worked it out, so it’s a bit moot, but I’d still like the community’s opinion on whether or not this homebrew feat is balanced for D&D 5e.
Inspiration taken from:
Sentinel (PHB 169)
Slasher (TCE 81)
(Sorry, feats not in the SRD)

This is for my lvl4 halfling barbarian; he has a STR of 17, so I really really want that sweet +4 mod, but taking the attr bonus is ‘boring’ to me. The purpose of this feat is to give him some more battlefield control (a la Sentinel; plus he took the Wolf Totem option at lvl 3), while also limiting the instances he can ‘do the thing’ (a la ‘Slasher’ feat)
Here’s what we came up with:

  • Increase your Strength or Dexterity by 1, to a maximum of 20

When using ONLY a melee slashing weapon* (with or without a shield):

  • When you use your Reaction to hit a creature with an Opportunity attack: if you don’t have disadvantage, roll a second d20– if the second d20 would also hit, reduce the creature’s speed to 0 for the rest of the turn.
  • If a creature you attacked on your most recent turn makes an attack against a target other than you, you may use your Reaction to make an attack against that creature

*we wanted to underline that this feat only works with slashing weapons, but not prevent a flaming axe or shields

How to ask for useful critiques?

As a GM, I strive to make the best experience for my group and adapt and improve my style and skill. I usually do this by live observation and asking for inputs after the session.

My problem lies in after session Q&A. Usually I ask them “Do you have fun? Which part is fun? Which part is not fun? How can it be better?” and they answer generally “Yep, it was fun. I like every part of it. I think it’s fine.”

I don’t want to look for problems that aren’t there, but I feel like I’m not improving the group’s experience this way. There are some parts where they are distracted enough to play with their phone – they are not engaged enough, so it’s obvious I can change some things, like pacing, so they can enjoy the session more.

I want my players to give feedback on every session. How to make them do so?

I’ve tried to make the questions as guiding as possible and as brief as possible, but they stick with the general “it’s fine”.

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