I think that the most important thing is to know what information you really want.
Questions like did you have fun? and which part was fun for you? are reasonably specific from a GM's point of view, but less so from a player's point of view. Players are a lot less likely to keep a running annotation of the game's narrative flow, structure, and the precise, planned elements which impact their fun.
I'm not suggesting that no players ever do this, just that for many players the game is a gestalt experience more than it is a simple combination of interacting, but discrete, parts. As a GM, the I'm keenly aware of all of the choices I've made in setting up a session, and how all of the events unfold during the session. Sometimes I have trouble seeing the forest because of the multitude of trees a GM has to be aware of. As a player, the only time I notice specific things that made the game especially fun or not fun is when they're really noteworthy. Not every session has such moments.
So while I think I understand where you're coming from with the what are the elements that impacted your fun at the table? questions, I'm not sure I agree that those are the best questions to ask at an every-session frequency, nor are they the questions that best guide you to what your players will want.
1. "Everything is OK" is not a useful critique
Some GMs, and particularly newer ones, have a tendency to feel a little bit insecure about their performance and want reassurance from players that the game really was fun. I was this way, earlier in my GM-ing career, though it may not describe you very well. But importantly, improving games through receiving player feedback will not come through identifying things the players found unremarkable. Questions which elicit that kind of response are likely to be mis-specified. I believe that generic questions about fun tend to fall into that category.
2. "Fun" is a conclusion, and a judgement that a GM can't necessarily directly influence.
Fun is the goal, not the method. The destination, not the route. Asking a player about their fun requires them to review the entire game, correctly understand the connections between things at a high level, and report that back to you clearly. And this is assuming that the game had specific moments which the players evaluate that way-- their fun may broadly come from playing an adventuring hero in a story that provides adequate set dressing for that.
More pointed questions are better. I like to ask my players, in advance, what sorts of themes and character developments they want to explore and then come up with content to satisfy those. When a specific session brings some of that to the fore, I might ask if session events are accomplishing what the player wants. This is still around the fun of the game, but asking what specific things the player is looking for helps orient the content, and then asking if the content is actually providing what the player is looking for. Asking about the player's perception of specific things in the game makes questions clearer and more pointed.
Finally, many players want to play the game, not design and plan the game. Not every player feels this way, but heavier influence on the course of the game and plot can detract from the novelty and fun of actually playing through the stories the GM creates.
3. If you perceive a specific problem, you can be direct in asking about it.
If you're concerned that pacing is an issue, ask about the pacing. I appreciate the desire not to confront players, but if they are taking out their phones and disengaging from the game you have strong reason to believe there is an issue.
Players may not even be specifically aware of the issue that caused their disengagement-- feeling bored is not the same as understanding exactly what might have bored them. Rather than trying to entice them to come forward with a specific complaint by asking what was not fun for them, it is often better to simply ask why they seemed to have tuned out for a bit.
Sometimes it's because another player has the spotlight, and there's little for them to do in the meantime. Sometimes it's because they're really engaged but are looking up some rule or other game-related information. Sometimes they're trying to manage a social situation and feel they needed to do it right at that moment, and the game was not even a factor!
4. Your players likely find answering these sorts of questions every session to be not fun.
Answering surveys is not usually that fun. Being compelled to do so is even less fun. Being forced to come up with specific items which may not even apply is not fun-- if they liked everything in the session, having to think up the thing that they liked the least just for the sake of having a direct answer to that question is not fun. The reason I believe your players do not enjoy this is that they are apparently refusing to engage, and try to deflect the entire situation each time it occurs. In a very real way, disregarding your players' responses to and apparent feelings about these "check-in" questions undermines the idea that you will be responsive to their concerns.
It's also worth considering that, the better a job you do of running games your players like, the fewer and smaller the complaints they'll have will be.
What's better?
What I have found useful is to have debriefings with my players after a distinct narrative section of the game: a campaign, an adventure, a notable subplot, etc. Anything that is self-contained enough that the players will have had a chance to see what you did and why, as well as have a completed experience to refer to when answering these sorts of questions. If players have an impression that's stuck with them, positive or negative, that's a good time to identify and discuss it.
Plots, campaigns, and gaming groups develop over time. Requiring feedback after every session is like demanding that someone judge a movie they're watching for the first time every ten minutes during the film.
If you really want per-session feedback, defining what you wanted to do, how you went about trying to do it, and how you wanted that to impact player fun removes most of these burdens from players:
GM: I wanted you guys to have the chance to solve a "whodunnit" mystery involving a group of NPCs, so I introduced those NPCs and then trapped you all in an area where you'd have nothing to do but solve that mystery. What did you think of how that played out?
Player 1: I like the idea, but the mystery we were trying to solve wasn't interesting to me. So, for this specific mystery, it was more of a chore than a game.
Player 2: I liked the mystery, and I liked interacting with the PCs, but I didn't feel like I had enough information to use to actually do any meaningful investigation. I felt like I was just flailing around until you revealed the next plot point, so it felt more like ordinary role playing for me.
Player 3: I thought the mystery was OK, but the fight at the end was a lot more fun! The mystery part took a long time to resolve, and then that fight felt rushed. I would rather have had less mystery plot, or have the mystery plot be more combat-like in some way, since that was the part I enjoyed most.
Focusing the question on how your hopes for the session matched against the experience your players had makes meaningful feedback easier to provide. Just asking about "fun" requires players to do a lot of reflection and analysis to even bring up the specific mystery plot, and then compare it to anything else that may have happened in the session to see if the whole session was fun for them.
