Suggestions for decreasing the size of Free Pro.

Maybe what you said can be realized. I don't know, but the cost may increase a lot, and it will be more expensive😔.

That makes no sense. As there is no prices available online and a PCB mounted battery would simplify production, which cuts manufacturing time and hence possibly cost. Even a more expensive battery could end up cutting costs due to increased efficency in manufacturing. A excellent  example of that is the Easee home charging stations who have a unique set-up for the PCB, where components are assigned front/back-side to decrease production time and  increace efficiency. 

As you know, Bluetooth and wireless are RF circuits, and the battery installed on the PCB may cause greater interference, which increases the difficulty of making. And in the process of production and testing, the disassembly of the battery also needs to be considered. Maybe,  is just my point of this. 

I don't think the other points are valid, but I do see the disassembly issue. Disassembly and recycling is a field I'm not having any competence in so can't  weigh in on any aspect, but I agree it's an important point considering the value and enviromental aspect. 

I did find this post from one of the manufacturer about the micro-battery talking about "hearing aids, IoT, medical wearables and true wireless earbuds." ( https://ensurge.com/blog-why-the-form-factor-of-a-microbattery-matters/ ). As they have medical wearables as customers I doubt the noise would be a issue. Might be a problem with distortian in real high-end hifi earphones, but tbh that's not really the end of the specter earfun is on. Again that's also purely hypothetical.