Learn Swedish: Tack för den stora hänsynen som du visar mig! - Thank you for the great consideration that you show me!

This is a classic example of Swedish double definiteness. When you use an adjective with a definite noun, you must include the standalone article (den, det, or de), add an -a to the adjective (stora), and keep the definite ending on the noun itself (hänsynen). You are essentially saying "the great the consideration".

Can I drop the word som in this sentence, like we often drop "that" in English?

Yes, you absolutely can! Because som acts as the object of the relative clause here (it refers to the consideration you are being shown), it can be safely left out. Tack för den stora hänsynen du visar mig! is perfectly correct and very common in everyday Swedish.

Does Swedish always use visar (shows) with hänsyn (consideration)?

Visa hänsyn (to show consideration) is a very common fixed phrase, just like in English. Another highly common combination you should know at this level is att ta hänsyn till (to take into consideration or to be considerate of). For example: Du måste ta hänsyn till andra (You must be considerate of others).

Is för always the preposition used after tack?