Got some pics.
The thermostat I'm using. Not super great but it's what I could afford at this time.
And the window right above it is a basement window well so there's no sunlight involved.
EDIT: the probe is inside the enclosure suctioned to the back wall of the tub. Tried putting it under the substrate but that didn't work well either, and just putting it out in the open means it'll just get pushed around inevitably.