I'm the opposite of Jean: I don't pay for anything before I need to, because you never know when a trip will be up-ended. The penalty for doing that is that I have to keep tabs on the websites of the most popular sights so I know when I can't delay my ticket purchase any longer.
You've already purchased the most time-critical ticket, the one to the Alhambra. The other popular sights in Spain do not sell out anything like that early. How many nights are you spending in each of the cities you've listed? That matters, because if you have a series of very short stays, you won't have a lot of scheduling flexibility.
Barcelona's particularly challenging in that regard, because there are so many places where you absolutely don't want to show up without a ticket, because you could be in line for hours. If you have too little time there, it will be tough even if you do buy tickets really early, because you'll have to guess how much time you'll want to spend at each sight, how long it will take you to get to the next one, when you'll want to eat lunch and how long that will take. To minimize the jigsaw-puzzle aspect of visiting Barcelona, it's smart to slot one of the tough sights first thing every morning. At least for that one, you know you won't have to drag yourself away from another sight in order to be on time.
I believe La Sagrada Familia is the Barcelona sight most likely to sell out early (though not like the Alhambra). If you want to ascend the tower, that might make things somewhat more challenging. Other places for which you'll want to get you tickets before the day of the visit (if you want to see them) include Palau Guell, Casa Mila/La Pedrera, Casa Batllo, and the Picasso Museum. The Palau de la Musica Catalana is also a bit tricky since the English-language tours can sell out. I haven't been to any of those places since 2016, so I can't tell you how many days ahead of time you should buy online tickets for each place. The first-time-slot tickets seem to sell out first.
The main ticket challenge in Seville is the Alcazar. I don't believe it sells out far in advance, but the ticket line will be long, and it's outdoors. The ticket line at the Cathedral is another you want to avoid, but Rick's guidebook explains how to do that by buying a combo ticket at the nearby Church of El Salvador. If you want to climb the Giralda Tower at the Cathedral, that probably must be arranged farther in advance. I have no knowledge about that; I am not a climber of towers.
I'm not sure about the ticket line at the Mezquita in Cordoba. Easter could well make a significant difference at any popular sight.
I wouldn't be concerned about the Cathedral in Toledo unless you hit it on a holiday weekend. That could mean a pretty significant ticket line (I ran into a bit of one back in 2016, and I wasn't traveling during the Easter period). I haven't checked to see whether you can even buy Toledo Cathedral tickets online.
In Madrid some people have run into lines at the Royal Palace. I don't know whether it sells out or it's just a matter of potentially getting stuck in a time-consuming line. However, it seemed to me (again in 2016) the majority of folks at the Palace were Spaniards. I'd expect the local-visitor count to be considerably higher than usual on weekends and holidays. If schools are out...
There's a potential for saving substantial money on some express-train tickets if you buy early, and Spain is a country where the fast trains do sometimes sell out. I imagine that's pretty common around Easter, so you'll need to keep a watch on the Renfe website next year to see how train capacity is holding up.