Tomato seedling leaves turning dark green and dying.

TLDR: What’s wrong with my tomato seedlings that I grew from seed? Worst case scenario I replace them with plants from a nursery, but if I have a disease/pest I don’t want to risk it.

I planted some tomatoes from seed about a month ago, and from early on the lower leaves have been turning a dark green colour and dying. The leaves are dying at about the same rate they are being replaced, so there are only about two “branches” at any given time.

The plants are indoors at a constant 25°C (77°F), and under artificial lighting (7am – 9pm). They also get a slight breeze from the A/C vents, but do not have a dedicated fan.

While the plants look very leggy, I believe this is mostly due to the leaves falling off before they get a chance to grow, and I think they should be getting enough light (although probably on the low end as the lights are just cheap Amazon ones that I purchased when I was starting out).

My primary hypothesis was that they were being overwatered, as the soil stayed quite damp for about the first week when I first transplanted them from seedling trays, but I have been paying closer attention since then and I think it’s okay now.

I’m in Zone 6 (Canada)/Zone 5 (USDA), and was planning on moving them outside in about 3 weeks. Despite the apparent legginess, I believe I can just plant them deeper and that shouldn’t be an issue. These plants will be grown in grow bags, not directly in the ground.

If it comes to it I can buy seedlings from a nursery, however I don’t want to do this if the issue is caused by a disease, as I worry it will spread from the soil to those plants too.

Any help with getting this under control would be much appreciated, and thanks in advance!