Subjunctive is a mood, and not a tense. "Were" is in a "past tense" and in the "subjunctive mood" or, more simply, in the "past subjunctive".

The word "were" is a subjunctive and not a past tense.

"Had" is better in this context For example "If I were a billionare who had lots of spare time, I would work in a charity shop for free"

had. Were is past tense so you need to use had which is past tense.