A better word is "tasallot/تسلط". The preposition is "be/به". Or you may use the related adjective "mosallat/مسلط": Hanuz be zabân-e fârsi tasallot nadâram. Hanuz be fârsi mosallat nistam.

You CAN use ( احاطه ) if you want to say what you mean, and Persians will understand what you are saying but when we want to talk about fluency in a language we use the word "تسلط" . Then your sentence will be; من هنوز به زبان فارسی تسلط کافی ندارم. (Or ; من هنوز به زبان فارسی مسلط نیستم) "تسلط" is an Arabic word but we use it too, that's why the second sentence I wrote is a little bit different. Here, «مسلط» is called " اسم مفعول" in Arabic. the meaning is somehow as same as «مفعول» in Persian. So you can use these 2 sentences. Hope this helps.

yes always preposition of " احاطه داشتن" is "بر" and this verb is not bad for this sentence ,but you can use better verbs . Good Luck