Assuming that what we saw from Douglas in New Orleans is what we are getting the rest of the way, the Pats have three starting WRs, Boutte, Diggs, and Pop. I don't think a 3rd WR is a big need. And Williams is getting open. Sometimes wide open. He just isn't on the same page Maye yet. If he gets this offense, he could be very good.
I think a third TE is far more important than you think. The Pats do not have a blocking TE. Neither Henry nor Hooper are good at it. It is hurting the Patriots' running game. I would rather the Pats pick up a blocking TE who can catch than a #3 WR. The Pats need to get the running game going and a blocking TE will be far more effective for that goal than a #3 WR.
BTW, the Patriots run a lot of 12 man personnel which has two TEs and only two WRs. According to AI, the Pats are in 12 man personnel 38% of the time and 11 man personnel (3 WRs) 48% of the time. So less than half the snaps on offense have three WRs. The #3 WR on the Patriots really isn't a starter.
I would only trade for a WR if they can get a legitimate #1 WR. Brown or Olave or someone else who comes up who we weren't expecting. The WR position right now is actually fairly good for the Patriots right now. Maye is second in the league in completion percentage. A lot of the advanced stats show that the Pats WR are above average in separation rate. Maye leads the league and throws over 20 yards. I mean there isn't much you can do to improve this unless you get someone elite or near elite.
Good insight on the playing time stuff. I was rough calculating it as Hollins plays about 50%, Douglas plays about 25% and they split #3 so one of them should be out there about 75% of the time. But Hollins is playing as the 3rd TE, Diggs/Boutte don't play 100% of the snaps, maybe they run 4 WR. That all adds up to 3rd WR play time not being as simple as just adding up those two players' snap percentage.
Still though, ~50% is the most used formation. I'd say it's a starter. I don't think Douglas or Hollins are bad, I think both are best served as #4 WRs. Douglas clearly has some great physical gifts but not consistent or reliable. That screams #4, package player to me. Same with Hollins, who does good things for you as a #4. I think you're stretching it making either of them a #3 WR that is part of your "base" offense.
It's ok. It's not the end of the world. It's not some crippling weakness that can't be overcome. I do think that Diggs is a mid to weaker #1 at this stage of his career and Boutte, is probably a borderline #2/#3, though he's moved up way more as a feasible #2 for me recently. And then you get to #3 where, IMO, it's two #4s combining for the role.
I'm all for any trade that improves the position anywhere up the chain. A #1 like you said pushes everyone down. Now Diggs is a top tier #2, Boutte is a great #3. Douglas/Hollins both get to to share the #4 role with their diversified skill set. If you get a #2 then Diggs is still your mid to weaker #1, but your depth char seems well-fitting from #2 on down. And if you get a #3, then you solidify from there. All improvement is good improvement IMO.
Also, "blocking TE who can catch" would be awesome, I agree, but that sounds like more than a #3 TE. Not sure who is giving up a player like that at a cost in line with what is appropriate for that role.
NFL trade deadline is usually uneventful without many/any impact players moved. I feel like a #3 WR and DE capable of playing heavy rotational snaps are probably the most logical "starter" positions where they could find someone. Both of those roles could be "starters" on the depth chart but really like 40-50% playing time. I doubt you're getting anyone more than that at the NFL trade deadline and those are two spots we could stand to improve a team that's 5-2 so I say pursue it.