The proper way to shoot slowly is to not use a damn big pipe! And large needles simply do a lot of damage and will lead to scarring of the muscles. For the quads #25 1" would work well, try injecting there.
In the gluts, you really need to know how to map the injection point. Best to mark it by pressing a pen cap against the skin, then swab that. You can really get hurt by injecting in the wrong spot.
If you have tight muscles there, and many do, massage those muscles! Massage the side that you will next inject in the days before. That fixed that problem for me when I was injecting gluts. I gave that up, and with my small TRT amounts, I now inject 28mg EOD with a #25 .5" .5ml insulin syringe.