How hard is it to submit an IG complaint as well as transfer from your unit before pcs date?

You can do an IG complain at any time- just make sure you have facts and figures- hearsay is not admissible. Changing units can take longer, you need to finf a slot in your MOS in the Bde, Div or post as talk to them about a transfer.

These are two COMPLETELY different questions. Submitting an IG complaint is exceptionally easy. Pick up the phone. Complain. Done.
Submitting an ACTIONABLE IG complaint is much more demanding. You must provide evidence of violations. Create and maintain a detailed log of inappropriate actions. Print problem e-mails. Write sworn statements. ask battle buddies who can corroborate to write their own sworn statements. Even then, the IG will run everything through the CoC to get their side of the story, and issue guidance to the CoC for corrective action. If the Company team is TOO far out of tolerance, the guidance may be issued to the BN to remove them. But often-times the guidance will be to the Company team to tighten up their shot group in this specific realm by doing that specific thing. Remember, the point of the IG is to fix problems, not to get people in trouble.
Now.... transferring from your unit is an entirely separate question. It is entirely possible that you can do an inter-post transfer. That requires an agreement between gaining and losing Commands. If you have a bad reputation (whether or not it is warranted) there may not be a gaining Command willing to take you on. Of course they might want to give you a second chance. If you have a stellar reputation, your current Command may not be willing to lose you. Of course, they might, for your own good. It is also possible you can get a compassionate re-assignment, although those are EXCEPTIONALLY rare. Compassionate re-assignments can be inter-post or PCS, depending on the nature of the requirements.

But here is the BIGGEST point I have for you. While being comfortable in your unit is obviously preferred, quite frankly, the Army doesn't care if you are comfortable*. It is not the Army's job to make you comfortable. It is not your CoC's job to make you comfortable. It is your job to adapt, improvise, and overcome. This is not an excuse for toxic work environments or toxic leadership. It is not a reason for the CoC to deliberately make things UNCOMFORTABLE. But on the overwhelmingly long list of things that a Commander has to worry about and ensure are done properly, making sure Soldiers are COMFORTABLE is not only not at the top of the list, it isn't even on the list to begin with. Whether or not you like your job, your unit, or how your Commander accomplishes the mission is irrelevant*. You doing your part to accomplish the mission IS. Focus on that.

*The obvious caveat of illegal, immoral and unethical orders applies. Do not let this be misconstrued as me saying it is OK to issue and/or follow illegal orders.

BMM OPFOR? what exactly is that?