How much Bahasa Malaysian I can understand after I finish the tree ?
Can someone advise
Grammar-wise, you should be fine since a lot of the differences are in the vocabulary. This Wikipedia article covers this subject greatly: https://en.wikipedia.org/wiki/Comparison_of_Standard_Malay_and_Indonesian
The intelligibility is not symmetrical. In my experience, Malaysian speakers usually understand Indonesian better than vice versa. This has more to do with exposure though (Indonesian pop culture is popular throughout the Malay world). In short, regardless of the region, there is little problem if you stick to Standard Indonesian.
As for the colloquial, good luck trying to understand anything even after finishing this course. Colloquial Indonesian and Malay dialects are wildly different from the standards, and you won't get much further without immersion. Keep in mind that colloquial Malay spoken in Malaysia are mostly NOT intelligible to Indonesian speakers without some dialect leveling efforts.
Almost fully, I'd say 85%-90%. Most of our Dutch loan words in Indonesian will be replaced by English loans in Malaysian. Other than that, I think Malaysian is just Indonesian but they replace every other word with a synonym lol. No Malaysian word have been too unintelligible for me as an Indonesian, and I believe this is vice versa with the Malaysians.
I understood a Malay word in Crazy Rich Asians because I know some Indonesian! A main character said a tiger in his house was named Harimau, which is the same word for "tiger" in Indonesian :)