thinking I will take the reflectors off the T5s and zip tie one upside down to one of the tubes to block it completely almost.
This seems a good way to begin - you can also alternate which tube you block if you feel some plants may need more/less light.
I've used Eco Complete in various tanks, hence my suggestion of enriched substrate 😀 ... but you sound a much more consistent doser (& likely aquarist) than I 😉