Exactly. 100% agree. Hate those phone mounts.
As for your question - its pretty darn costly. I am in same boat as you. Here's my take on this: The updates offered are a load of BS, YMMV. About 18 months back exit # changed near where i live, important highway. Around Q4 2017 updates rolled out. In my Mazda this change was NOT reflected. In my Garmin and my friend's TomTom - it was reflected. To be noted - Garmin uses NavTeq same as Mazda.
So - having no updates - in reality you won't loose much. What I plan to do is hold of for 2 years. In between the interim updates - I agree will miss but frankly I am 200% certain won't miss much.
Within this period Mazda will get its ship in order and roll out AA / Carplay / etc. Outcome: those darn SD cards will become very cheap. Also - supposedly as and when Mazda does roll out compatibility - its saying it'll be backward compatible but some small h/w might be required. Again - I'd rather wait for that than buying a subscription.
In eBay there's dime a dozen of these SD cards available < $100. However, parsing this and other forums - it's a hit or miss.