Bus/Train Ticket Question

Re Trains - You might find this article from The Man in Seat 61 helpful. (scroll down for the "when" discussion which includes possible price advantages) There is no one answer to when is "best" to buy train tickets. Intercity train tickets tend to be best bought early for price advantages whereas regional trains tend to not have price differences. Except when there is a sudden sale, or another exception arises, or above generalities don't apply. Some country by country differences also. In other words, each situation can be different.
https://www.seat61.com/european-train-tickets-online.htm

Another good article on when ticket sales generally open by country:
https://www.seat61.com/european-train-booking-horizons.htm

Buses - I haven't had much experience. I have noted that when I've researched a bus route as a "Plan B", it is helpful to download the applicable app and I can monitor ticket availability.

Adding: My personal preference is to buy as tickets come available for distance trains and day of, usually by app, for regional trains. Always interested if someone has a "better" system!

Flixbus has service throughout Europe and their model is super cheap bus fares the earlier you buy them. A good website to check out ballpark bus fares and schedules is www.GetByBus.com
High-speed trains in France and many other countries follow the same business model— the earlier you buy them, the less expensive they are with “Loisir”
tickets on TGV trains as little as €25 when bought as soon as they go on sale several months in advance of the train’s departure date. Walk-up fares for the same trains can be as high as €150 depending on the demand. So, don’t wait to buy them at www.SNCF-Connect.com
Belgium and the Netherlands offer discounted day return tickets ( called “ Dagkaarten” in The Netherlands) which make sense for daytrips. They are restricted to non-peak hours, usually outside of morning and afternoon commuting times.
There are many other discount train ticket schemes in Belgium and the Netherlands which, as referred to previously, can be researched at
www.Seat61.com
Just to get an overview of train ticket prices in Europe for the cities in an itinerary, I go to www.TheTrainline.com to check prices and schedules before buying them directly from the train company operating the trains. If buying them straight from the source becomes a problem due to internet platform issues, I just buy them from the Trainline’s website. The national train companies contract with The Trainline to sell their tickets, so it is a reputable site.
Have a great trip!

In case you are thinking Eurostar between London and Amsterdam, more info for you:
https://www.seat61.com/trains-and-routes/london-to-paris-by-eurostar.htm

I have always purchased Eurostar tickets directly from the website as soon after release as I know my schedule. Fares go up as lower prices sell out:
https://www.eurostar.com/us-en/train/london-to-amsterdam

If you are thinking of trains in Belgium and you are a senior, there are spectacularly cheap day trip fares, one cheap fare to anywhere in Belgium.

I'd echo the sentiments here that purchasing tickets between the countries should be done decently in advance. Unless you require special seating arrangements, don't bother paying extra for reserved seats. I took a month-long trip around Europe and was on about a dozen or so trains—only once was I sitting in a seat someone had actually reserved. When that happened, I simply got up and moved back a few rows to another open seat until I reached my destination.

FlixBus is a super economical option and could be worth comparison shopping. Sometimes the bus price is significantly cheaper than the trains, but remember that the bus is traveling on the road like other cars, and any accident or construction can really slow things down. I remember taking a trip from Bratislava to Prague on FlixBus—about five minutes outside of Brno, we got delayed for two hours because an accident was blocking the entire highway. I think if Brno was my final destination, I would have asked to get my bags and walked the rest of the way!