Planning one trip routinely runs to ten hours or more, and a good share of that time produces something the traveller is not happy with anyway. The hours do not mostly go into deciding where to go. They go into rebuilding the shape of the trip every time one detail changes.
A spreadsheet is the usual answer and it half works. It holds the facts and knows nothing about how they relate, so when the flight moves it tells you nothing about what else just became wrong.
Record these four things per booking
This is the whole method. Everything else is detail.
4 things to do
Ticks are for this visit only. Nothing here is saved anywhere.
The fourth one is the whole point
A transfer is not merely a thing at two in the afternoon. It is a thing that exists because of a flight landing at one. A hotel check-in is not just a time, it is downstream of the transfer.
Write that relationship down once, when you book, and you never have to reconstruct it. Skip it, and every disruption starts with working out from memory what was connected to what, usually in an airport. The method is set out in what else your trip depends on.
Do not build a full itinerary
An hour by hour plan for a two week trip is a document that is wrong by day three, and rewriting it is where most of those ten hours go.
Record the fixed points, which are the things with a booking reference attached, and leave the rest genuinely open. The fixed points are the only part that breaks expensively when something moves.
One place, not six inboxes
Confirmations arrive across several email accounts, a couple of apps and occasionally a screenshot. That is fine while nothing goes wrong and useless at six in the morning at a desk.
The references are what you actually need under pressure, and they need to be somewhere you can read them in three seconds without searching.
Assume one thing will change
Something will move on almost every trip with more than three moving parts. Planning for that is not pessimism, it is the difference between an inconvenience and a ruined day.
The practical version of planning for it is simply having recorded what depends on what, before you needed to know.