Yes, that’s why the DC-powered OpenSprinkler is shipped with a 7.5VDC 2A power adapter (it’s included by default for US/Cadana orders, but not included for other countries). It provides a holding current that’s the closest to a typical sprinkler solenoid.
I think 12VDC is still pretty safe: I’ve seen discussions online where users have been using 12VDC to drive their sprinkler solenoids for years without any problem.
We had a batch of DC-powered controller that used 2.5mm plug (more specifically, the plug is 5.5mm OD x 2.5mm ID). The current batch should all be 2.1mm, which is more common. The power adapters that we ship can accommodate both 2.5mm and 2.1mm.