I don't understand why you use pouvais in this context. For me, si je pouvais means if I could in the sense of in the past. Therefore it should be:
si je pouvais, j'aurais été astronaute.
If you want to say if I could in the sense of something for the future, shouldn't it be:
si je pourrais, je serais astronaute
As the saying goes in French, " les 'si' n'aiment pas les '-rais' ". If you wish to talk about a hypothetical situation, 'si' (if) is followed by L'Imparfait + Le Conditionnel Présent. Here is a link to the lesson explaining this exact rule: l-imparfait-usage-in-hypothetical-clauses-introduced-by-si-if-followed-by-le-conditionnel-present
I hope this is helpful.
Bonne journée !
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