I could well be wrong about the AAC passthrough, and I should have hedged that statement with "allegedly" as I've not tested it myself.
To your other point though, I disagree - there are plenty of ways you could pass through an unchanged AAC bitstream, but still mix in other sounds when required. For example, having the sender duck the original bitstream out temporarily and send a mixed replacement bitstream while the other sound is playing. Or (and this would only work if you control the firmware on the receiver, but if you're using Apple headphones with an Apple device, that's not a problem) sending multiple bitstreams to the receiver and letting the receiver mix them.
I wouldn't say the Pixel line's hardware is rubbish, more that Google is focused on having a polished "it just works" experience rather than trying to differentiate themselves by having the fastest, biggest, newest hardware in the Android market.
The mobile market hit the "diminishing returns" point quite a while ago and for a lot of people - probably the majority - the only reasons to upgrade are security updates ending, or because a non-replaceable battery is getting to the end of its life.
I used to upgrade every 12-18 months religiously, but now my Pixel 5 is coming up on 3 years old and I'd happily keep it another few years with a battery replacement, if the updates weren't going to end shortly.