I visited the Expo the last week.

The weather is indeed brutal when there is sunshine (the air remains very humid), with 1pm-4pm being the worst time. It's less bad when it's cloudy, but still hot as hell. An umbrella is a must. I never used an umbrella against the sun before, but I bought one on the very first Expo day because it was obvious I would not survive otherwise. The weather became bearable after 5pm.

There are many shaded seats under the big loop, but for me, the key to staying cool was to visit air conditioned areas now and then. There are less crowded pavilions (notably the Commons), there are shops, there are places selling food, there is an air conditioned dedicated rest area (East of the Bahrain pavilion IIRC). You have to drink plenty of cold drinks, there are vending machines with cold green tea throughout the site (reasonably priced at 180JPY per bottle).

Yes it's less hot under the ring, but the problem is, you have to stand in lines to visit pavilions. A few lines (e. g. for Singapore) are under the ring and thus in shade, but most lines are out in the sun. Very few pavilions provide shade for people standing in lines.

Another important item is to have a small folding chair or stool. It makes waiting in lines much more bearable. I got mine from Tokyo Hands (for like 3800 JPY), later saw much cheaper ones in 100yen shops (don't remember if it was Can Do or Daiso).

The advance reservation lottery doesn't help much. I played on 4 days, won Blue Ocean Dome once (not exactly the most exciting pavilions), didn't win anything else on the other days. There is also an ability to reserve pavilions 3 days before the visit, but the website gets very overcrowded, more often than not it will force you to wait in "line" for an hour or so, and when you finally enter the website, all availability will be gone.

The only advance reservation that did work for me was the same day reservation after entering the Expo site. If you get the 9:00 slot and enter before 9:30, there will be pavilions available for reservation (I got Australia and Future of Life this way).

Early morning: if you manage to enter the Expo site before 9:30am, you can get into the Japanese pavilion with almost no line.
During the day: none. Other visitors are not stupid, pavilions worth visiting get crowded.
In the evening, lines get much shorter. Around 8pm, you can get into Germany with a 20-30min wait time (2 hours during the day). Note that pavilions start closing the lines at that time.

Very high. People with reservations are let into the pavilion in groups, so you are not free to enter as soon as you arrive. They will form a line, and then you will all enter together. Get an umbrella - there is no way around it.

A few random notes from my Expo experience:

The most important thing to aim for is to get a 9:00am time slot for entering the Expo site. There will be still long lines at the entrance (at the West gate, lines took about 20 minutes). But when you enter the site, there will be relatively few people there, meaning shorter lines. You can run to the Japanese pavilion, enter with almost no line, and afterwards visit another popular pavilion with little queuing. You will also be able to use the same day reservation (only available AFTER you enter the Expo) to get a reservation. Before 10am, it has plenty of availability.

If you only got a 10am time slot, join the line for 9am time slots but make sure you only enter the security check after 10am (go backwards in the line if needed). This way, you will beat all other people holding tickets for 10am.

If entering through the East gate, use the rightmost line for security (it moves 3x faster because it ends with 3 security checkpoints, while every other line ends with one security checkpoint). At the West gate, the rightmost line is slightly faster too (due to vacant security checkpoints for disabled etc. handling some of the people from that line).

Taking a train to Sakurajima and then a shuttle bus to the West gate requires and advance reservation for the shuttle bus. Some websites claim you don't need a reservation for them, but that's no longer true. I never tried using the East gate for entrance early in the morning, an acquaitance did and reported waiting 20 minutes in line to enter the Expo site, which is similar to the West gate.

The best thing to do between 2pm and 3pm is to have a late lunch at an air conditioned place. The heat is brutal. I mostly used the Japanese food hall to the North of the West gate.

There are two conveyor sushi bars on site (Sushiro and Kura Sushi). Both are reservation only. It is nearly impossible to get a reservation for Kura Sushi, but for Sushiro, you can usually get a reservation if you visit them before 11am. Then you have to constantly check on their website and you must come to them when your reservation number is about to get called. I visited the Kura Sushi flagship store in the city instead - found it disgusting (too little fish and not yummy at all). Sushiro is decent low end sushi, their "sushi bar of the future" at the Expo is an interesting experience, with a virtual conveyor belt etc.

The drone show was at 7:57pm when I was there, it was OK but nothing exciting. On many days it was canceled due to wind. All water shows were canceled due to legionella in the water.

The pavilions I liked the most were: France, Germany, Japan, Portugal, Qatar.

The US pavilion has a separate line for English speaking tours, it's shorter than the Japanese line (ask the staff). About 70% of the exhibition is about NASA and space exploration.

The staff at the end of each line knows how long the line currently takes - don't be afraid to ask. Some pavilions have a higher throughput than others, so the length of the line does not always say how long it is going to take.

The Food Truck area to the southeast of the "Forest of Tranquility" has great soft melon ice made with real Hokkaido melon - a concentrate of happiness for me!

When you leave the Expo through the East gate at the end of the day, there is a 20-30min queue to enter the subway station. Leaving through the West gate isn't much better though, because you will spend like 15 minutes in a shuttle bus packed like a sardine can.

Get the "Expo 2025 personal assistant" app - it has a usable Expo map that shows your location.