First of all, I doubt your premise that quality time can replace quantity of time. I get that getting time with one's children is a privilege that not everyone enjoys and I don't mean to suggest that those who don't can't be good parents with close relations to their children. And by all means, if quantity of time is limited it's probably preferable if the time you do get is high quality. But if at all you do have a chance, when it comes to building relationships, I think few things can ever beat just showing up, through good times and bad, to share the exciting parts of life and the mundane alike.
I'm guessing how much time you have access to your child is more or less out of your control, but things being what they are, I think it's still a useful mindset to have.
Doing fun things in the weekend is great. You're expanding his world, giving him new experiences, new points of reference in life. Those are all important parenting objectives. But in those moments, his focus will be on the exciting new experiences - not on connecting with you. That happens in the downtime between the fun things.
Think about the people who are closest to you, who you open up to. I'm not surprised if what makes them stand out from the rest of the people in your life who are not as close, is the sheer amount of normal dull life or perhaps even hardships you've endured together; not that the trips you've taken with these people are so much better, etc. Again, not to shame you for not having more time, but rather to remind you that this time also matters, and matters greatly. Perhaps what you should do moving forward is rather to lower the pressure to always make the best out of all the time you do have.
I think this is a great deal of the explanation for why you're seeing these changes at this point, and I really hope that home isolation will be a time for many families to reconnect as you seem to have.
I also have to say, the question "how was your day" is rather problematic as a means to connect. It's like asking "how are you" to an acquaintance, a courtesy phrase where both parties know that you're just supposed to say "good, and you?" regardless of what's going on in your life. Even if your child has had a rough day, bringing it up might be hard, and he's not likely to spill all of that because you asked a question out of the blue.
If you want meaningful answers, you must signal that you're actively interested and have time to catch whatever he throws at you, and you weren't just making small talk. You also need to make sure he feels like he's in a safe space to share. I find that happens when information is not demanded, but a door is opened for sharing, should he want to. Saying what you observe and what you think about it is also a good way to show that you're seeing him and not just asking routine questions, and at the same time it frames the discussion. So instead of "how was your day", you might try "sweetie, I see you look a bit down today. We all have days like that, but if there's anything in particular that's troubling you, know that I'm always here for you. Sometimes it helps to just talk about it." Or "my, you seem to be in a good mood today, has anything happened? Did you find money on your way home?" (or something phoney like that, which might elicit a response like "no, it's just that..." By your own proposed answer you're anchoring what level of specificity is appropriate in a response.)
Make good use of the nuggets he has thrown you during this time. Don't ask questions that just demand a daily report. Engage and connect with him using the information you have. "Hey, what did teacher X do to his problem pupils today? Did he get mad at anyone?" The things your child has already shared are a good indicator of what's making a big impression on him right now, and being able to talk about it with at least some little insight makes you part of the in group. Also, in these areas he has already decided to share a little, so the threshold for sharing more will be lower. You can then use that as an entrance point to unravel new information. Perhaps you'll notice that a few names recur commonly when talking about the unruly pupils, and you can have a discussion around that. Then next time you have that information to draw from too, and you may ask "hey, what trouble did pupil X get themselves into today?" Or perhaps in talking about unruly pupils, he'll disclose some incident that'll give you new insight into what your kid is dealing with at school.