Reina Sofía question

Not sure about this strategy, but I can speak to the opposite one, which is to go
during the free hours from ~7-9pm.

You don't purchase tickets in advance. Instead of going in the main entrance in
the plaza, go to the side entrance on Ronda de Atocha. You'll beat most of the
crowd, and while there still may be a crowd of people around Guernica, if you
retrace your steps around 8:30, you will likely have the painting to yourself.

We were there in mid-August 2024. I had pre-purchsed tickets for opening time. We were first in line for visitors with tickets. There were a couple dozen people who needed to purchase tickets in a different line. We went straight to Guernica. Upon arriving, I was surprised to see that there was already a tour group there. They were not in either line at the front of the building. They had obviously entered through a group entrance with their guide. So don't plan on being ahead of the crowd even if you pre-purchase tickets and are first in line.

That being said, Guernica is large. Probably much larger than you are anticipating. There is plenty of room to see it. Also, there are a ton of other great works of art right in the same wing, so if it is crowded when you first enter, hang out and view the other artwork and come back. It's easy to see if the crowd has thinned out because there are 3 large entrances to the room. I returned two more times and took another look. The third time there were only a couple of other people there. That was about 45 minutes after we first arrived.

Long story short, either way I think you'll be fine if you go right at opening. Be sure to look ahead of time at the route you need to take to get straight to Guernica.

You don't need to book tickets in advance if you arrive as soon as it opens.

Just get there before it opens by 1 minute (south side of the museum on Ronda de Atocha), go straight to the ticket booth, get your ticket, walk across the courtyard to the entrance, show your tickets and go through security, take the elevator to the second floor, show your tickets again (IDK why you have to do this), and then walk to the room with Guernica.

If you move with purpose and do all of that, you will be alone in the room with Guernica for a few minutes. I've done this several times without booking a ticket, it has never failed me.

Thank you shoeflyer, sibdaisy, and Plectrude. Your answers are very helpful.

All good advice above, but also consider prebooking a private tour (see website) for the main artworks = Guernika takes some explaining.
Also, the entrance is currently through the Sabatini building, due to works.

In mid-September, we walked in via the main entrance on the plaza early afternoon. There was no line, no wait for the senior tickets. There were others viewing Guernica but, as @slbdaisy said, the painting is huge and it is easy to have an unobstructed line of sight. Also, be sure to see the sketches on the opposite wall. We found them fascinating.

Was just there today. They had a seperate entry gate for the people who qualify for free entry.