Lastly, is Tabari's tafseer available in english?

Not as far as I'm aware. There's a one-volume translation that takes you part way through the Sura al-Baqarah (i.e. the first sura of the Quran after the incredibly brief Fatiha, opening verse), so basically unfortunately "no."

academically credible books of tafseer

All Tafsir are at least in theory works of scholarship but they are also religious works of scholarship. There's not really such thing as a secular academic tafsir, and different works of tafsir have different purposes. One might aim to explicate the meaning of specific kinds of verses, for instance ones with legal ramifcations, or to explain the meaning of the text more generally. They also vary by sect and outlook, so there are Shia tafsirs and Mutazilite tafsirs and Salafi tafsirs and so on.

All of these might be studied for one purpose or another even if they're not especially useful as textual or critical analysis of the Qur'an itself. For instance, one might study Sayyid Qutb's tafsir In the Shade of the Qur'an to understand Qutb's religious views of the Qur'an, even if the work itself isn't a revelatory work of scholarship.

If what you have in mind is something more like a critical edition of the Qur'an, unfortunately there is nothing like that available for reasons that I've discussed on the sub before here and here.

I haven't read it but it sounds like something close to what you might be looking for is The Study Quran which is religious but more ecumenical and the product of a modern academic tradition than the classical medieval tafsirs like Tabari, even if it is still religious in outlook.