We had our SFH in Northern California vacated a couple of months ago and it was roach infested with German and American roaches. We initially used pesticide smoke cans and they did kill many but not all. Then we purchased Advion gel bait tubes and applied it at all places it could be consumed by them but it didn't help much as it dries out quickly. Next I've sprayed Boric Acid all around the baseboards, cabinets, under the stove/oven and refrigerator.
I still see a few around.
What should I do next to rid of roaches while the house is empty before the next tenant moves in?
Those roaches make bunny rabbits seem downright celibate by comparison. Females lay egg cases that hatch into legions and legions of babies that grow into like numbers of adults, which lay more egg cases, and . . ..
All the materials you've already tried will work and appear to have worked, but I suggest one more thing in addition:
"Growth regulator" or growth inhibitor. It's a hormone that, sprayed in the areas the roaches run, prevents baby roaches ("nymphs") from maturing to adulthood by stopping them from shedding their skin, and stopping the reproductive cycle. The really great thing also is: it only hurts the bugs, not you or your pets, and it works on fleas, too. There's no odor, it won't stain things. Just spray it, as much as possible, where it won't get washed off by water.
The only downside is that it takes a bit of time. But, it works. The one for "german" roaches will work on "American" roaches, too. Murder 'em all.
If you feel really desperate, you can tent the house and fumigate it with methyl bromide or like chemical. That kills EVERYTHING. Including roach egg cases. But it's costly, and MB might have been banned.
I'll bet that if you do a good job with the inhibitor, the roaches will stop appearing. That said, if you want to try and plug drains, etc., can't hurt.
Let us know what you do and how it goes.