Read the TN to GC thread by CuriousGeorge in the top of this forum (sticky)
https://forums.immigration.com/threads/curiousgeorge-tn-gc-link.339271/
There are several examples of people on TN going to GC through marriage.
The key point is you need about 1 year before TN expiry, and you cannot leave the USA for almost one year. Not for any reason, vacation, business trip, visit sick relative, etc. If you leave USA, your I-485 will be considered abandoned. (yes, this is silly, but these are the rules)
You need to wait at least 90 days from last entry to USA before filing I-130/I-485/I-765/I-131, then you get letter for biometrics appointment (fingerprints/photographs) at ASC, then wait for your EAD/AP before you can travel again. Once you have EAD you tell employer you are now working on EAD and not TN. Since COVID, the ASC for Biometrics have really backed up, and taking much longer (they were closed March to July).
You can check current USCIS processing times for filing forms here.
https://egov.uscis.gov/processing-times/
There are other website that provide much more info for marriage based GC. For example, the evidence to provide to the Green Card in person interview at USCIS. (joint bank statements, wills, beneficiary designations, etc), the medical process, etc.
https://www.visajourney.com/guides/
If you are willing to spend money on a lawyer, why not talk to them before you get married. (after doing your own research on the internet).
As you go through the process, please post your experience on this forum for the benefit of others.