Yep, only liquids should be measured in volume, since liquids do not compress

Yep, everything in weight. It works so well.

There is a Polish website https://kalkulatorkuchenny.pl/, where you type, say, 1 teaspoon of sugar (łyżeczka cukru) and it will convert it to mass, volume, spoon and number of glasses. I'm pretty sure, there is an English language alternative, but didn't find any

For using volume measurements (weighs are still superior tho) flour shouldn't be packed in but spooned into the measuring device and leveled with the back of a knife but brown sugar should be packed into the measuring device.

In recipes, they'll call for a heaped teaspoon or tablespoon, everything else is implied to be leveled, especially leavening agents like baking powder/soda. There's also an understanding that certain things don't need as much precision, like adding in flavoring extracts.

I also do really like the nice even 25° increments that recipes align to for farenheight.

I assume flour can have a lot of moisture weight to it, which may change depending on the location or season. Weight is still the better measure, but still not perfect.