You're right, I meant to say deduct insurance premiums, not deductibles. If only!
You couldn't switch plans in 2018 without paying a penalty for having non-ACA health insurance.
Now with the individual mandate (tax, penalty) gone, you can switch halfway through the year (or earlier) to a non-ACA plan (short term health insurance plan).
I believe if you only have your HSA plan for 6 months, you only get to contribute half the allowable amount.
It's important to make sure you have the HSA eligible plan starting in January, and pick it out before the December 15 open enrollment deadline.
With the short term plans, you can pick them whenever you want. They have plans for 3, 6, 9, and 12 months (possibly more).
The way I see it, if you have any HSA plan starting in January for 6 months, and nothing comes up where you have to pay some of the deductible, jump on to a short term plan that's 6 months.
The short term plan will be lower monthly premiums, PLUS if you have something really bad happen during that 6 month period you're close enough to the open enrollment period to jump back on the ACA plans (covers pre-existing conditions).
I think this strategy is the best of both worlds. The HSA plans are more expensive, but you get them during the beginning of the year when you have 12 months ahead of you of something bad happening. Once you're 6 months in, given that you've been healthy, you switch to the short-term plans because now you're closer to the open enrollment period for the ACA plans that cover pre-existing conditions.
I don't recommend going without health insurance, but for self-employed, healthy people this seems to be the best route to get covered, make some HSA contributions and not pay too much in premiums.
Hopefully somebody else can chime in if the plan above isn't a good idea....