@edavison & @alsart, folks, if you wish, all the research that needed to be done on this topic of preparing panels for oil painting has been done here already, look for thread named; Priming - Oil Painting, Acrylic Gesso, Plaster of Paris, Traditional Gesso, Rust-Oleum etc., posted by @ Kaustav in April 2017, 4 pages total.
I work from dark to light, establish darkest darks first. For brush handling, get used to get comfy holding it naturally, making one deliberate single stroke at a time, wiping the brush clean, picking up a fresh clean dab very stroke to keep colors bright and clean. Concentrate, focus and breathe, slowly and surely. I believe the idea is to not leave it to chance and have confidence.
You are amazing! I like your paintings here and this exercise that you share with us. I like the velocity and passion at which you are working, this is motivating and strong. This is enough to shake out any cob webs accumulated in my studio. I believe this is more important to the artist's soul and keeping it alive, if the 20 minute painting works out, then bonus. In these paintings you conquered subtle, getting values real well, limited palette is great, and developed a greater sense of realism. In 20 minutes , it may be wise to work with few values such as 3, 4. Your composition is not horrible, it just wasn't considered carefully enough beforehand on those tries. Just as you have to have your palette already to go, preparation of composition ready to go as well, this will help you to slow down and relax and boost your confidence. I disagree with no way of finishing a painting in 20 minutes, take @ Kaustav's recent Teddy Bear painting as just one example, but it doesn't always and rarely happens, but what an incredible moment it was. I believe this is what there is to live for in these kinds of exercises, that element of surprise, when and if, one works out. The ability to purchase a roll of canvas at a deal is great (just check it real well before purchase for quality control ) and use it as you need as you go works real well. Good work, keep painting!