I still say that heated air is being lost somewhere.
There is no way in hell that room should not be damn near 80 degrees or more on a good full properly burned load.
When you say cold start, did you purposely let the stove go out, or just didn't have enough coals to reload on top of?
I still have at least 2 hours worth of coals to burn down, that I just drug forward, and as usual, I loaded at midnight.
Not only should you be getting plenty of heat, but if your loading before bed, you should have plenty of coals in the morning.
Unless your burning soft wood at that size you have.
Our members in the West and PNW, parts of Canada etc, only have soft wood, pine, fir etc, an they have no issues of getting 8+ hours with coals for reload.
The way I see it you have two issues to solve:
1. The need for truly dry decent sized splits to load and burn, which quite frankly, you don't have in the pile I see.
That would be scrap starter wood for me &/or used to finish filling in the top of small gaps in a reload.
2. Determine where the heated air air going, where the heated air is being lost, and sooner or later you will have to put a block off plate in, there is a very good chance your losing heated air up that cavity, only to be bled out to the wild blue yonder, and being replaced with colder air dropping in on top and around the insert. Neither F/G nor Rockwool are air barrier, and do not stop air flow.
Nest time the stove is "cold", take the surround off and put your hand around the top and sides, see if you feel a cold draft flowing on top &/or around the insert.
Keep the garage door shut. With that open, you might as well have the front & back doors to the house open.
Once you get your present issues squared away, then youi can work on figuring how to heat the garage.
If your going to leave the garage door open, then you really are handicapping yourself and the insert.
You have a learning curve, we all have had that at the start. You have many fine folks giving you some sage advice. Be patient with learning how to load, what to load, how to burn, and apply the pointers the members are giving you. This time next year, you will be chiming is as guys are here. They had issues the first year, learned the wood, the stove, hones their burning practices, and are now tell how this year is so much better.
Many of us have been there. Hell my problem in the start was my insert burning too hot. Came to find my stove did have some Q/C issues. But still got it figured out, and 9 seasons later am still honing my burn habits, loading strategies, and how I operate the stove. After my first year, and getting in tune with the insert, I have made large to small adjustments each year, and have constantly heated with less wood each year, while not sacrificing anything.
Your first challenge is decent wood. .