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What I suggest that you do is a load calculation on your own. Do this before you make a final decision on either feeding the new garage or/and the extra power needed for the addition.
Doing this load calculation will give you a bit more clarity on how much power you will need and how much extra power you should have. Negate the option of either running the garage from the house main panel or a new service just for the garage. The load calculator does not take into consideration that fact. The load calculator will just let you know what will be needed overall. There are a lot of load calculators on the Internet. Find one that is easy to use and understand. They all perform the same function but some are more user friendly for DIYers.
Here is one to start with. Learn what VAs are compared to Watts. Understand the basics of Ohms Law so you can input the proper information. If something does not have the watts listed on it then take your volts times you amps that would be listed on the item. If the item has its volts and the watts then to find the amps you would take the watts and divide it by the volts. You don't have to do this with all your appliances but the bigger ones. Heat pump, hot tub, dryer, hot water heater any large appliances in the garage such as your EV charging stations etc. Also figure how much power you want for your addition. Yes, the bathroom may only require one 20amp circuit by code but you may want more than one circuit going to the bathroom. Example: your wife uses a curling iron. It has to heat up; while it heats you decide to use the blow dryer for your hair - there goes the breaker. You want to install a ceiling heater/fan/light unit, that will require a separate circuit. Running a few circuits to the bathroom will help you with this. Your new bedroom - may want a window AC. Run a separate circuit just for that. Running a window AC and a vacuum cleaner on the same circuit will more than likely trip the breaker.
I am not attempting to complicate your decision - I am attempting to ask you to educate yourself -
read this as example. Don't rely strictly on someone else's word as to how much power you will need compared to what you have and what you may want in the future since this is your permanent home. This way when you speak with your electrician you can have a visual picture of what you need.