Start with reading good English. It is usually by people whose English is 1st, or close to, language and who have both yeshiva and college education. Brits use more words than Mericans.
Rabbi Sacks Z’L? Rav Avrtaham Twersky Z’L – less poetic, but well written.
In terms of writing, Jewish schools do not teach it well more often than other subjects. To teach math and science, one would need some qualifications. English is sometimes seen can be taught by anyone who can speak that language and is not otherwise occupied … So, start with a small classics – Strunk “Elements of Style” and then do online classes, maybe Khan Academy. If taking academic course, be careful, as English teachers tend to be very aggressive in pushing their ideology onto students – more likely progressivism, use some obscure Arab or African literature, sometimes their religion.
PS one writer asked a more successful one what is his secret. He asked back – what do you do eary in morning? Of course, I am writing! And I am crossing it out.