There isn’t a plateau at any point, as soon as your hormones are above physiological you should be able to make continuous progress, given diet and recovery are perfect ! A plateau is most of the times when either one these variables (training, diet and recovery) isn’t on point and you’re either undereating, Training too much and have too little recovery capacities.
In terms of dosage if you made great progress on 300 mg for your first cycle then there is absolutely no need to go straight to 500 for your next cycle. After a 8+ week TRT phase you can make great progress again at 300 mg per week. Maybe increase it to 400 for the last 4-8 weeks of your second cycle but start with 300 again absolutely !
I am a fan of increasing dosage over the course of a cycle so if you aim for 4-450mg peak load per week for the last 8 weeks of your second cycle and you plan to run 12-16 weeks ( blood work and health markers like blood pressure should dictate when to stop a cycle), then you may start at 300mg per week for the first 4 weeks. Then for week 5-8 you go to 350mg, then for week 9-12 to 400mg and it blood work is good and all other health markers too you can go to 450mg per week for week 13-16 and then go back to a TRT. This should give you plenty of progress again if you eat enough and rest enough and training hard.

I would recommend you also to increase your injection frequency to keep side effects as low as possible. Try every other day at least, using insulin syringes and preloading them makes to easy and less painful