If Company A is 30% equity, 70% debt currently, but will have a capital structure of 50% equity, 50% debt in year 5, which capital structure do you use for your DCF?

The 50/50% capital structure would be used.

When calculating WACC, we assume the optimal target capital structure, which is the capital structure the company will have in the long term and the optimal capital structure for a company in that industry.