FWIW, I did a 5-day trek in 2000. No need to book in those days and not too crowded on the route.
But on their final day, all the trekkers wanted to be at the Sun Gate for sunrise, so it was pretty crowded there.

But what I would say is, if at all possible, don't miss out on climbing Huayna Picchu, the mountain that features in most photos of Machu Picchu.
Reaching the summit is a terrific reward, worthwhile and really completes a Machu Picchu visit. But bear in mind only about 4 or 5 people can sit on the summit at a time so be prepared to wait your turn on the slopes just below.

Also, great care is needed. There are tricky sections, and steep narrow stone stairs with nothing but air on one side, where any slip or trip would be serious.
A lot of renovation work on the climb to the summit was being done in 2000 so it may be better now.

If you don't fancy the climb, go round to the north side of Huayna Picchu where you'll find massive stone works and caves, (Temple of the Moon) all overgrown and partly hidden (in 2000), that is very atmospheric and straight out of Indiana Jones.

When I was there, there was a small manned gate that lead to Huayna Picchu with a signing-in/signing-out arrangement.

Yes, exactly. I think it mainly affected us 'older' trekkers. One night the nausea was so bad I made sure I could get my head right out of the tent, to leeward, at a microsecond's notice! (But it was never needed thankfully).

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TTR250 - London to Cape Town