Research is useful up to a point. Past that point it becomes a reason not to go.
The goal of research is to arrive knowing enough to make decisions, not to have every decision made in advance. Some of the best experiences on a long trip come from a conversation with someone you just met, or a sign you noticed from a bus window, or a recommendation from the guesthouse owner. None of those appear in a guidebook.
That said, there are four things worth researching properly before you travel: flights, accommodation, a handful of specific sights or experiences you know you want, and local transportation.
Flights
I use Google Flights for the overview and Skyscanner to find combinations that Google misses. For long-haul travel I look for flights with one stop and a reasonable layover, at least two hours. A 75-minute connection through a large hub is fine on paper and a disaster in practice.
Booking around six to eight weeks out tends to hit a reasonable price point. Any earlier and you are paying a premium for flexibility. Any later and you are paying a premium for scarcity.
Accommodation
I use Booking.com almost exclusively. My target price is around $25 per night, which gets you a clean room with a decent shower in most of Asia and a more spartan option in Europe. Read the recent reviews and sort by proximity to where you actually want to be, not by the algorithm’s idea of a good deal.
A useful rule: book the first two or three nights, leave the rest open. Once you arrive and get your bearings, you will have better information than any website can give you.
Sights and local transportation
For sights I read a couple of travel blogs and one or two posts from experienced independent travelers. I skip the listicle articles. I want to know what something actually costs, how long it takes, and whether the crowds are manageable.
For local transportation I look for the options that locals use. In Sri Lanka that meant the train. In India it meant a mix of trains and local buses. In Vietnam it often means overnight sleeper buses. These options are slower, cheaper, and more interesting than private transfers.
The point is to arrive with a framework, not a schedule. Know what you are interested in, know roughly where you are going, and leave enough space in the plan to change it.