I would never let a client purchase in certain areas of my local market....

Until the lease ends and then the legal notice time in your market has passed.

The STR market here has become way more saturated and competitive especially in San Juan. 

Two properties available in the highly sought-after Gresham Park area—one on-market, one off-market.

So it could take a decade to get to todays market rent. 2) it’s almost impossible if not impossible to non-renew a tenant, especially if you’re not an owner occupant. 3) As a seller, even if I could remove paying tenants I never would without a non-refundable payment from a potential buyer that covers all vacancies for 6 months if they fail to complete the purchase. 4) You MIGHT say as a buyer that’s worth it.

You could put most of your funds in the stock market and access them when needed for a HML.

There are really great REI opportunities in different markets - markets in the Midwest and Southeast of the country are particularly lucrative - where not only are the purchase prices reasonable for most RE investors, but the homes are turnkey (new builds or completely rehabbed homes, tenant ready, systems 10 years of life remaining on them, property management teams in place) with appreciating home value AND appreciating rent.

Since I'm based in Tampa, I don't look outside and compare markets. 

What strategies have worked best for marketing midterm rentals in secondary or smaller markets like Portsmouth?

Much like other markets in the Midwest, investors go to our market because of the low barrier to entry.