If this is a hardware product (ie not installing a new OS on an existing phone), you first need to establish a supply chain with somebody who will sell you phone SoCs (unless you have a >$100m to make your own). That will limit the field very considerably.
I know MediaTek is a bit more friendly to small volume OEMs compared to say Qualcomm (and forget about Apple or Google). But MTK is terrible about keeping their software stack up to date, so any kind of support lifetime goes out the window.
The other option is a non phone SoC (Rockchip, Allwinner,NXP) plus a separate modem which is what the PinePhone does (crudely). But there you have to craft more of your own OS/driver stack, you aren’t getting it on a plate like MediaTek or Qualcomm give you. And you can’t reuse much from those.
And this just gets you an Aliexpress grade phone running bare AOSP, with a lot more hardware/software engineering to get even level with the current state of the market.
The other way is to go to Shenzhen, take whatever cookie cutter phone platform they have to offer (MTK or worse), and then deal with supporting it better than the manufacturer. That’s what Fairphone do (with abandoned Qualcomm silicon) but they can’t fix vulnerabilities in their binary blobs.
Any way you do it it’s hard, very hard.