Yes, you can connect Garmin Connect to Strava and your workouts will be automatically uploaded to Strava. Or you can download the Stava app directly to your watch.
You can upload your own music in MP3 format, see https://www8.garmin.com/manuals/webhelp/forerunner245/DE-DE/GUID-CD4439DF-46FF-4279-A8D5-8DA61C87A4EB.html.
According to Garmin (https://www8.garmin.com/manuals/webhelp/forerunner245/EN-US/GUID-BE888A02-503B-41E6-8F34-15604E0B9CA3.html)
it is 3.5 Gb for approx. 500 songs
Strength activities can be recorded. In addition to the normal values such as pulse, calorie consumption, etc., reps, exercise type and weight can also be recorded here. However, the input via the watch is rather impractical.
Other sports available are Bike, Run, Treadmill, Indoor Track, Pool Swim, Bike Indoor, Yoga, Walk, Track Run, Track Run, Trail Run, Row Indoor, Cardio, Pilates, Breathwork, Stair Stepper and other. The sports can also be expanded at will.
I use the watch for running and road cycling.
Yes, the watch shows the speed directly, for running as pace in min/km and for cycling as km/h.
There is no specific mode for windsurfing, but you can create your own individual mode and name it windsurfing. What the watch should display in the mode, e.g. km/h or other, can be selected.
there is no other garmin watch that has exactly the same range of functions as the 245 music + pay. the most similar and with garmin pay are the garmin forerunner 645 and the garmin venu - all differ in nuances. the best thing is to determine the most important functions for you and then see which of the watches fits best. or go all out and buy the forerunner 945 - of course much more expensive.
there are many tests and comparisons of the watches on the internet, e.g. on youtube. look around and you will surely find what you are looking for.
I hope I was able to help you a little :)