>>bscphi+(OP)
Right, but those are your specific requirements, the next person will insist on dual-sim. LineageOS capability is another piece that requires work, the headphone jack might be too hard for a small-scale small-phone manufacturing line as well, since someone else will require that it's waterproof.
There is a smartphone that meets the needs of a small-phone purchaser, after all, if small is the requirement - the iphone mini. But purchasing that would require some compromise on your hardline requirements, which will be different to someone else's hardline requirements such as a swappable battery.