I also like cold frames and sunny windows. I'm in FL, so a greenhouse would be pretty useless, but over the winter I start stuff early on our screen porch (w/ vinyl windows).
My single best piece of plant propagation wisdom -- tomatoes do very well from cuttings. In my experience, this isn't very widely practiced, but I swear by it. After a plant is nearing the end of it's run, I cut off 8 inches or so of "fresh" looking end growth, strip the bottom couple leaves, and plant it. I tend to use a very unscientific mix of 1/3 mature compost, 1/3 perlite, 1/3 old soil. I like to plant these seedlings in 4 inch pots (so I don't need to replant them immediately) and then plant them in 3-5 (or bigger) gallon pots w/ the same soil mix. Last time I did this (February) I had 100% success! In the past, I've lost ~25% of the cuttings. I'm currently doing cherry tomatoes (supersweet 100) and mid-sized (patio).
Another plant that is great for the home garden (depending on where you live) and is dead easy to propagate is katuk. It is my go to green. Grows like crazy, easy to harvest, easy to propagate. You need to be somewhere warm, though. I'm in zone 9A.
I highly recommend looking into permaculture techniques. I did a permaculture design certificate online through Oregon State University and it was great.
I also highly recommend looking into local extension offices. Down here, we have IFAS, which offers workshops (w/ free goodies) on rain barrels, micro irrigation, composting, etc.