You are getting good advice and it sounds like you are doing fine.
First and foremost, take the clamp off your heat lamp and throw it away so you are not tempted to rely on it. Firmly attach that lamp so it can’t fall. Two supports are better than one, with one being a backup. I use wire. Plastic can melt. String can burn through or melt. Wire can break if it flexes back and forth. You don’t need to start a fire by that lamp falling or getting knocked down by the chicks jumping.
I don’t use this chart at all but I’ll still give it. It’s not a bad place for people with little or no experience to start. This chart is extremely safe. In reality the chicks can do pretty well with lower temperatures if the brooder is really draft-proof, if you have enough they can huddle together and generate heat, or if you have bedding they can cuddle in. But since everyone doesn’t have the same conditions or the same brooder, these numbers keep everyone safe, even the ones with a lousy brooder.
0 to 7 days – 90 to 95 degrees
8 to 14 days – 85 to 90 degrees
15 to 21 days – 80 to 85 degrees
22 to 28 days - 75 to 80 degrees
29 to 35 days – 70 to 75 degrees.
After 5 weeks mine generally don’t need heat. They’ve gone through nights with a low in the mid 40’s and even down to the mid 20’s before six weeks age. But a key to that is that I keep one end of the brooder warm enough and let the far end really cool down. Mine are acclimated to those colder temperatures because they play all over the brooder and only go to the heat when they need to warm up. My brooder is in the coop from Day 1.
Another key is that my brooder in the coop and grow-out coop are really draft proof. The chicks are not going to be hit with a cold breeze. Part of when they can go without heat is how acclimated they are and how your facilities are set up.
I much prefer a brooder with one end warm and the other end a lot cooler. Not only does it acclimate them, I don’t have to worry about keeping the entire brooder at one perfect temperature. Let the chicks go where they are comfortable. That means a larger brooder.