Km, secular studies was advocated way before moses Mendelssohn, who you for some reason call rabbi.
The ramchal says to teach children secular wisdom, and the Gaon says that “to the extent one is missing in secular chochma, that is the extent he will be missing in Torah”
The rishonim advocated it as well, including the rambam who says that studying teva brings to love of Hashem. The chovos halevavos implies that it was common for yidden to have secular books as well.
This is all predicated on the assumption that nothing in the books is apikorsus. Modern orthodoxy allows reading apikorsus to a large degree in the interest of open mindedness, depending on how modern they are and how not Orthodox they are.
Rav hirsch repudiated reform and the likes of mendelssohn.
You’re vastly overstating mendelssohns reach; he had his circle of naskilim, but klal yisroel were not moved by him very much, and in later years his books woild fall into obscurity, preserved mostly by bloggers who are obsessed with deviant jewish figures.
For the record, shadal had a bigger impact on jewry than mendelssohn, and he came earlier. He influenced Italian jewry towards secular things, and had some ideas which were apikorsus, but he was still not as divergent as mendelssohn.