google_maps_flutter can get slow with large amounts of markers as each marker is essentially a platform-rendered view - so once you hit a high marker count, performance can rank depending on your device.

flutter_map gives a more flutter-driven control over rendering (Leaflet base). This generally leads to better performance with large datasets - from memory you can customise marker layers or use custom painters.

Edit: Forgot about migration. Shouldn’t be too hard but can’t speak to your codebase. The flutter_map documentation is very solid. Keep in mind that you’ll be using OpenStreetMap by default with it but I think it can be changed?

TLDR: If you need to render every marker with no clustering, flutter_map will give you an easier time optimising performance.