So I would upgrade the T8 to T5HO. Unfortunately if you raise the T8 then he will get hardly any UVB. T8's produce much lower levels. With a 5.0 bulb in a single bulb fixture he would need to be within inches of it to get the right UVI level. If you upgrade to a T5HO fixture with a single bulb and get either a 5.0 or 6% UVB bulb then you would be able to drop basking to 9 inches below. This would give the correct UVI for him of about a 3. Get a 24 inch fixture to run the entire width of the middle of the enclosure and have a basking branch down below the fixture. Take a look at the first graphic on this page. It will show you the difference between a T8 6% and a T5 6%. Both would be equivalent to a 5.0 bulb.
https://chameleonacademy.com/chameleon-cage-set-up-replicating-the-sun/
Now with the basking light. Is it by chance a focused heat bulb? The kind that indicate it is a spot light? If so these produce much hotter focused beams of light and heat. If that is what it is I would switch it out for a standard incandescent bulb.