Two things you can do without buying a meter.
Use a phone light meter app as a rough guide. It measures the wrong quantity, so do not treat the number as authoritative - but it is excellent for comparisons. Measure at the top of the tank and at the substrate and look at the ratio. In a tall tank people are routinely shocked by how much is lost. That comparison alone tells you whether your problem is the fixture or the distance.
Pick plants for the light you actually have. This is the cheaper direction and it works. If you cannot get strong light to the bottom, plant the bottom with things that tolerate low light and put the demanding ones high - on the background, on branches, near the top. Layering by light requirement is what makes tall vivariums work, and fighting it by trying to grow a sun-lover at the substrate is a losing proposition regardless of fixture.
Things that generally cope with less: many ferns, pothos, some philodendron, mosses if the humidity is right. Things that generally do not, and are commonly sold for these tanks anyway: most bromeliads, anything with strong red or variegated colour, and most succulents.
Given three failed rounds, I would do both - better light and a more forgiving plant list - rather than betting the fourth attempt on one change.