How early are you planting out? I just posted in another thread about starting seeds too early = not good. I grow zinnia every year, and 6 weeks is plenty of time under lights to get them to transplant size. Maybe the type of light set up you have makes a difference if your experience is zinnia take a long time? Do you ordinarily keep the lights that high above the trays? They should be only a few inches at most above the plants (and seed trays) for best growth. (not trying to be critical -- just trying to understand why this has been your experience).

You can see how close I have the lights above the germinating seeds on the second rack, it's only about an inch. The lights generate enough heat that I don't need a heat mat. You can also see the plants on the top left rack and some on the bottom rack are reaching for light because the lights are higher there than they should be (can't always fit everything perfectly by height -- it's a juggling act as things grow).

My downstairs setup looks more like yours, actually. This is only the germination heating pad in my office (and the lighting is used for illuminating plants in winter). I put in a photo of the growth racks below from early last April. Plants generally go out for hardening off in very late April to May first, for planting around May 6th to 10th.

Most of the zinnia will start in mid to late March.

Aaah, that makes sense. I thought the photo in your original post was your main light set-up. You're able to plant out earlier than me. I aim for 3rd-4th week of May but wait on some heat-loving vegetables until 1st week of June -- depends on temps, they can vary widely year to year.

That is a sweet light cart you've got! Did you make it yourself? Mine is only 4 tiers, I'd love to have an additional couple tiers (would probably need a step-stool to reach the top LOL!) but mine was purchased and I can't add onto it. I do love it, though. I had wanted it for so very long, finally got it as a graduation gift from DH when I finished my master's. :0)

I like to keep my friends close, and my germinating seeds closer. :-) There are light decks in the bathroom for my desert roses, downstairs in the den for a variety of plants, and the cellar for the sprouts.

I built that out of a basic five shelf rack that had enough space for what I needed, plus that top rack that can take two lights supported off the cellar beams for taller things. It works like a charm, but by the end of April the tallest zinnia tend to be buried in the lights. With T12, they're cool enough that it isn't a problem. They're on chains so I can drop them down to nearly touch the flats when the plants are tiny.

Congrats on the Master's! A friend of mine just finished hers, and then got COVID-19 from defending her project... And then got pneumonia from the COVID. She's fine.

Thanks. Yea, I had wanted to move for a long time but DH didn't (he's a plopper...) so finally made a bargain with him that we would move when I finished my master's -- I think he agreed because he didn't think I'd finish LOL!! Got a nice light cart out of the deal, too :0)

(but now that we're here for a handful of years, he couldn't be happier - he loves it here and the larger property we have)

Sorry to hear about your friend. COVID sucks.