If you are being targeted and/or thinking of quitting you have three options. You should consider the first item even if you think things are great.
Document everything with audio recordings, notes in real-time or immediately after, emails and other materials. Move all to devices and locations that you control and assume that you will only have access to some emails and messages via discovery. That will be difficult and time consuming compared to having it in-hand. Do not engage the ombudsman or HR. The ombudsman is fairly impotent and will simply engage HR. HR will only seek to defend the company by dismiss you or getting evidence against yourself. You are not a lawyer. Don't think HR helps you. They work against you.
Number one with written responses to HR for factually erred reviews, warnings or other official communications within one week of your receipt. Take your time responding, but be direct and have someone else read it first. The argument is that, if you're a good employee, you will diminish the company's single-handed efforts to create a negative narrative against you. Their goal is to protect from litigation. Your goal is to make certain they don't control the case. Be honest with yourself, but don't let mind games to make you doubt yourself either. What managers and HR write doesn't have to be true. Your responses, on the record, immediately, and to the company makes them contemporaneous and the company has a chance to respond. This is far better than trying to respond years later. It raises your profile though. You have to decide if you want a bigger target on you.
Engage counsel. They will advise you to follow item one immediately. Each state is different with limitations and sometimes hostile to the employee labor law. Your objective will be to leave with money. You don't want the job, nor will that be an option, if it gets to this point. Resolution will take the form of a demand letter where you hope the company wants to get rid of you quickly. Having solid records and actionable claims helps. For example, if you are told you are doing great and then your review is negative you have reason to explore discrimination or other causes of action. They can fire you in most states for no reason, but when they lie to make a case it isn't for no reason anymore. If it goes to arbitration or litigation, find a firm that works on contingency.
This isn't legal advice. It's guidance that doesn't hurt. If you are pushed out you want them lay you off with severance or have the best case to arbitrate or litigate. You can win and they settle. You should be ready to legal up and don't let them bully you into a lost reputation or rights.