– No, I haven’t seen optical DO probes less than 350, but I’ve only used PME minidots (onboard logger and should fit under a dome; ~1k) and Hach Hydrolabs (multiparamater sondes that are likely too big for domes; several K). Electrochemical/Clark-cell DO probes may be another option (calibration will drift more). If you only need a few days for chamber estimates (benthic productivity?), the calibration of electrochemical probe may be good enough.
– For the arduino systems, typically I have a separate voltage regulator for the sensor than the board. Note, this preference may be lack of arduino board use however.