What visitation limitations can come from a Canadian visiting the US for too long in the year?

Background

I've recently entered a relationship with a person from Canada. Part of that commitment involves visiting frequently throughout the year; this commonly involves me driving to pick her up, as opposed to flying to save cost, so she can stay with me in the US. She's planning on coming up to visit again for the entirety of the Summer (four to five months).

Extra information in case it has any impact:

• She is 20 years old.
• She is a third-year university student and is committed to her program for another two years.
• She has visited the US three times already this year. Twice to visit me (~five weeks in total); once to visit her friend (five days).
• We've both lived in our respective countries all our lives.
• I've only stayed in Canada with her for a few days at a time.

My Knowledge

^{Feel free to edit this information if any of it is misleading/incorrect.}

• The 180-day limitation mentioned in these threads [ 1 ][ 2 ] are for people who visit in one consecutive stay; not for totaling 180 days in the year.

• She will be considered a resident alien of the US if she crosses the 183-day mathematical formula, as noted on this AmericanLaw website:

  A foreign national will meet the Substantial Presence Test if he or she is present in the United States for 31 days in the current year and the sum of the following:

  • sum of the days present in the United States in the current year,
  • one-third of the number of days present in the first preceding year,
  • one sixth of the days present in the second preceding year equals or exceeds 183 days.

  If a Canadian meets the Substantial Presence Test but is ineligible to meet the Closer Connection Exception (i.e. since he or she is present in the United States for more than 183 days in the current year), he or she will be considered to be a resident alien of the United States. Resident aliens are taxed on worldwide income in the same manner as United States citizens.

Question

• Are there any common limitations, mainly regarding stay durations or frequencies, that could make further visits throughout the year difficult or impossible?

Also, I fully understand that a visitation could be rejected for many reasons and that there's no guarantees regardless of how long of stay. I'm simply trying to avoid breaking any rules or missing any legal obligations that can come up from this international relationship. This is all foreign to me— pun was most certainly intended.