Prebook Madonna and Child ticket Bruges? and weather question too

I can't imagine you would need to prebook tickets for this! Just by them there. our first big trip to Europe in April of 2005 we started in Belgium and worked our way south. First two weeks of the trip were rainy and cold. We brought rain pants and I was very glad we did. We wore waterproof hiking shoes which I was glad for also.

We were in Bruges fall of 2023. We did not have issues with rain. Consider wearing leggings and carrying rain pants to wear if necessary. We felt Bruges was very, very busy. We were there over a weekend. The breweries we wanted to visit were a bit of a challenge. Also the boat rides on the canals were busy. Somebody on the forum suggested starting at the spot by the fish market. (It is under/close to a bridge). We did not wait at all for tickets and it was a short wait for the cruise. It was easy to get into the museums and churches. It seemed like many of those on the streets did not go into the sites.It was not at all busy at Church of our Lady (with the Madonna and Child). I believe we may have gone there right away when it opened in the am, and it did become a bit busier, but I did not see any line for entrance to the church.

My opinion on weather forecasts. Longer range forecasts are subject to great error. Even shorter range forecasts can be less than accurate. Also, the weather forecasts I follow tend to be on the pessimistic side. Also know, that if it shows rain for a particular day, it could be that it will rain for just a short time or during the night. I wouldn't stress and you can't control the weather anyway.

ADDED: if you foresee rainy weather, for footwear, consider Allbirds Mizzles. They are water resistant and warm. LOVE them. I have since purchased them for my son who lives in Seattle. He loves them, too.

“AND I have been checking the weather forecast in Belguim and Amsteredam, it has been raining for weeks.”

I’m curious to know what your source is, because I live here and it definitely hasn’t been raining for weeks. Most days were overcast but without rain and the temperatures in March have been higher than normal.
It’s impossible to predict the weather 2 weeks in advance, but for next week rain is indeed predicted. This is perfectly normal weather for April, because weatherwise the month of April is notorious for very changeable weather. It can be anything from rainy to sunny and dry to storms with hail. The weather in April is so changeable and unpredictable that we have a saying about it; April doet wat hij wil. This literally translates as “April does what it wants”.
So you should be prepared for every type of weather.

I was in Burges in October 2023. Like Jules, I also saw a good number of people on the streets but relatively few inside the excellent Groeninge Museum or the nearby Our Lady church. There is free entrance to the church; you only pay to get close to the Madonna and Child sculpture. There was no line at all for the ticket and maybe a couple other people viewing it midday when I was there. April is probably busier but it is hard to imagine needing advanced tickets.

If you look closely at the weather forecasts, many of the days that show rain say less than 50% chance and/or only in the morning or night -- not all the time. If your plans are somewhat flexible, in Amsterdam, there is always a nearby museum or other attraction or cafe to duck into.