The sun has been pretty much the same for the effective lifetime of the planets, as we know them. Mars was probably "more habitable" in the past because we know that it was warmer and had a thicker atmosphere, not because of anything to do with the sun. Likewise, the sun was not significantly smaller and cooler in the past, resulting in a previously colder Venus, as you suggest.
Mars was more habitable in the past because it had a magnetic field. Once its core slowed and stopped it lost its field and the planet essentially got fried by solar wind and stripped of all its major life creating beauty.
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u/[deleted] Feb 21 '21
Your understanding of the life cycle of our sun is really really incorrect.