I used these from Max Gain systems for an OCF once used. These are excellent quality with easy thumb-locks for each section. They do come in a heavy duty version and different lengths up to fifty feet and they have lots of needed accessories like guy rings and easy to use guy rope tensioners.
[URL unfurl="true"]https://mgs4u.com/fiberglass-push-up-masts/[/URL]
However, these are really meant for dipoles and other light weight antennas where there really isn't a lot of weight at the top of the mast. If done right, I believe it could support an Antron 99.
I would definitely attach the mast to the peak of your garage roof. That's how I once did mine but the roof peak was only about 14 ft so I still had to run 2 sets of guys but that was for a 50 ft mast for an OCF antenna. I also used 2 wall mast brackets and a eave mount upper bracket. The mast was just resting on the ground, no concrete.
At your roof height, I believe you should be fine but I also would use a lower mast wall bracket, maybe two like in the pic below. That could end up being be a steady mast braced at 27 ft and 3ft of the extended mast for 30ft overall.
One thing to consider about fiberglass masts, Unless you buy the 45 ft or 50 ft version, your top sections will be thinner with a 30 ft mast. With the 45 or 50 ft, you can utilize just the thicker and heavier bottom sections to get up to 30 ft. This of course adds to the costs. The top sections narrow down to 1/2 in and that isn't enough support.
Another thing to consider, Does the Antron require a DC ground? No DC ground on a fiberglass mast of course.
You may just want to consider using metal pipe. It'll be cheaper and no doubts if it will support your antenna.
Years ago when I ran an Imax 2000, I used 10 ft of 2 inch galvanized pipe and slid a 10ft 1-1/2 pipe inside about a third of the way and drilled holes and put a grade 8 bolt thru it to secure them. This made a nice telescoping mast so I can easily bring down the antenna if needed to work on it. I still used the same wall mount and eave mount brackets to secure it to the house.
2 inch Galvanized pipe is available in 21 ft lengths.
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MGS Pushup poles.
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