Simply send her notice and move on. likely a good thing you found it out now.  

I do  have a good paying job that helps me pay my primary residence, but I like to plan for rainy days.My ultimate goal is have enough rental income to live off of, so I know conventional wisdom says I need to buy more rentals.

For me 5% or less is generally when I would not pay off early. 3.2% Do not pay that off early.  

I have, at my sole discretion, waived the late fee as a one-time courtesy to a tenant whom (a) is in good standing and has paid promptly for at least a year, (b) has informed me in advance that the rent would be late, and (c) pays up within a few days after the late fee would have kicked in.Whenever rent is not received, I start right away with a rent demand letter and proceed with the eviction process on the first day it is permissible by statute, even if the tenant eventually pays (including collection costs). 

They are not well funded, so pay as you go is important to them. 

Right now they’re living rent-free, so there’s little motivation for them to move somewhere else and start paying rent, even if you hand them cash.Eviction timelines vary a lot depending on the state and even the county.

Does this tenant usually pay on time?  

Did they pay you all of your contractually agreed upon late fees?

It seems after covid and 'eviction protections' a fair number of people have learned they can game the system and just never pay/stop paying for months and expect that if they pick back up paying all is good.  

You’re in a great spot with lots of options.From a tax perspective, your STR could qualify as a non-passive business if the average stay is under 7 days and you manage it yourself.That opens the door for a cost segregation study and big bonus depreciation write-offs often opportunities that many CPAs who don't specialize in real estate miss.Like others have mentioned above, you can’t do a Solo 401(k) with just W2 income, but if you treat the STR as a business and pay yourself for managing it, that income could qualify, giving you another powerful retirement planning tool.Next steps: start by finding a tax strategist or CPA who really understands real estate and short-term rentals, and an estate planning attorney to protect what you’re building.