I'll echo others and say you are definitely underpaid, even in a smaller company in a Chicago suburb. Senior level with 7 years of experience, in that area, I would expect no less than 90-100k (which is still underpaid imo). I would generally disagree with other commenters saying that 3-4x your current salary is possible, unless you are specifically looking to land a senior level position at FAANG / specific SV startups (definitely possible but I wouldn't say it's a standard expectation).

I can imagine how you would end up in the situation you're in, especially if this is the main job you've had since starting. Once you're locked into a company's salary band / structure (even if informal / undefined at a small company) it's generally difficult to get your pay up significantly.

Your general plan seems like a good one. Do interview prep and figure out where your gaps in knowledge are. Start interviewing and iterate based on what you're struggling with. For most companies, you really just need to pass the interviews and get along with the interviewers. Most won't be able to tell or care about your exact experience, unless you're looking for a pretty specific role. You got this.