Like how websites such as Grammarly is used for English, are there any grammar checker for Malay? All I found via Google are some spellcheck websites.

No.

Tatabahasa changes so often it's hard to have a website to pick up.

When I was in school. "di kalangan" is wrong, "dalam kalangan" is correct. "Sehubungan itu" is wrong, "Sehubungan dengan itu" is correct. Among others.

If you read Malay newspapers nowadays, many don't follow the basic grammar you learn in school.

Google are not always correct. Use other methods for self improvement.

I guess writing with correct tatabahasa is not really that important, or maybe they just didn't realise that they were wrong.

Just ask nicely any of your melei friend.. I think they will be happy to help you

True. And other examples are
Hal ini kerana - salah
Hal ini demikian kerana - betul
dari masa ke semasa - salah
dari masa ke masa /semasa ke semasa - betul

But Malay newspapers don't really care for these updated grammar. And people still use adalah/ialah interchangeably.