Semi-colon or comma here? I am available Monday starting between 8 to 10 PM based on your location; with open availability on Sundays?

Use a semi colon to separate dependent clauses.

I would change the whole structure to read more like this:

Based on your location, I am available on Mondays from 8:00 a.m. until 10:00 p.m., and Sundays from [time] until [time].

Note: I put “based on your location” first as it separates it and removes it from the middle of the section about your time. The comment about Monday and SundayS was correct. Are we talking about multiple Sundays/Mondays or just one? Double check that. As for the time, I removed the words “starting between” because it is inconsistent with the “8 to 10 PM”. Also you put 10 PM but not 8 AM in your original version, another inconsistency. Furthermore, I would highly recommend that you give specific times for Sunday, since you did so for Saturday, and 8:00 AM to 10:00 PM is basically “open availability”… You may as well put a time for Sunday too unless you want this person contacting you at 3 AM.

neither; should be a comma

Comma. The second phrase is a continuation of the first.

i would use a comma for sure

A comma would suffice, but it is also necessary to make the days parallel. As worded, it refers to a single Monday, but multiple Sundays. That incongruity makes it difficult to understand.

location, with an open availability on sundays?

Put a comma before "with open availability..."

I would rephrase it to this >

"I am available starting Monday between 8 to 10 PM based on your location, with open availability on Sundays."

Coma, because there is no change in thought.