I've gone back and forth between post cards, yellow letters, professional letters, etc. and have been pulled in different directions with different mailing companies. 

Mixing STR and LTR can work in a building like that.

You need a pitch to win in a personal letter.

There are obviously a lot more factors that go into it - real estate is always going to be a mix of the tangible property/market itself with a mix with the investor/operator including experience, risk appetite, financial situation etc.I would say however if you are just getting started in real estate - LTRs are probably (in a vacuum) the better bet as STRs are certainly more sophisticated and challenging from an investor perspective

Less expensive (smaller) GC with mixed references.

You are terminating their lease, which is done without court involvement.I recommend you present the tenants with an introduction letter as soon as you close on the property.

Is there a legal letter I should send?

I'd consider starting with letters then perhaps following up with a few rounds of postcards.

You could respond in  a letter with those points and their options. 

Mixed usages, such as a furnished mid term rental of one unit or vacation rental usage (where permitted) is also an excellent way to boost revenue.