Voltage drops

greetings to all. The car is Prius C 2014.
There is a constant drawdown of voltage - 12.4-12.8V on the voltmeter.
The battery was changed a year and a half ago for some kind of non-name 35 Ah acid
There are third-party consumers in the car - two "class D" audio amplifiers. I don’t listen loudly. one amp feeds a small subwoofer (300RMS), and the second feeds front speakers (80RMS), the subwoofer does not bang and, does not shake the window.
the voltmeter shows 13.6-13.8V, regardless of the volume of the music. When switching to "D", it immediately drops to 12.8.While the parking lights are on, the voltage starts to normalize. Tried to drive a couple of days with the music switched off, and still the same crap. The problem is observed both in the cold season and in the warm season.
Is the battery dying or is there a bigger problem?

 

1 - When the HU is off all amps are off as well, as they are all related to remote coming from HU. Sometimes I drive with HU off but woltage drops are present.

2 - Idk how to measure consumed current, but I saw priuses with much more powerful audio systems and no woltage drops were detected

3 - Good idea but thinking if replacement of the 12V battery to new AGM with bigger capability could resolve the issue since the +1 battery gives additional weight