The problem is it won't run any apps, so you'll need to carry this open-source secure phone in addition to your normal phone.
Thankfully, some apps have both web and native mobile versions but for a modern digital life, the critical apps are sadly not on both versions.
Unless you're Fabrice Bellard who literally created a 4G softmodem - no. It takes a whole lot of people (or, again, one genius Fabrice Bellard clone) to design a smartphone. You'll need AT THE VERY LEAST:
1) a SoC that has reasonably open device drivers and specifications - without that, all attempts are moot
2) a hardware engineer to deal with the PCB
3) a low-level system engineer to deal with the initial bringup (aka, porting u-boot and maintaining it)
4) an RF engineer to deal with the black magic that is designing ultra high performance PCBs that deal with the RF stuff (2G-5G phone networks, BT, WiFi, NFC, GPS) and high-frequency buses (storage, RAM, baseband, USB, PCIe, CSI/DSI)
5) a GPU driver engineer of the class of Alyssa Rosenzweig to get the GPU drivers to behave (she literally provided better-compliant drivers than Apple)
6) a battery engineer to ensure you don't end up with something like the ill-fated last Galaxy Note (that had to be fully recalled due to battery issues)
7) a ton of software engineers to get the basic things running that people expect from a smartphone (e.g. phone calls, 911, SMS, MMS, a browser and enough userland libraries so that third-party developers can begin to port games)
8) hosting engineers that deal with reliably delivering OS updates, application updates and A-GPS data
9) a skilled purchase and finance department to acquire all components as well as skilled QA people to make sure you don't get screwed in your supply chain by someone cutting corners or trying to engage in outright fraud
10) plastics and metal design engineers for the housing and other related engineering, and you'll probably also need engineers specializing in mass production and assembly as injection molding is a skillset on its own
11) engineers specializing in low power domains to get something that doesn't eat through the entire battery in a matter of hours
12) UX, UI designers to get something people can actually use (partially, that's also compliance stuff - think of accessibility laws)
13) testers to test your device against an insane load of other things - headsets, headphones, consumer and enterprise wifi, car head units, mice/keyboards, game controllers, USB hubs, monitors, projectors, adapters, dongles, IPv6 in its various abominations, phone network-side vendors, how devices behave in trains, cars, airplanes, cruise ships, in temperature and humidity extremes, under water, in back pockets (bending!), in dirt, dust, rain, being drenched in all kinds of beverages, muck, snow, fog, right next to extremely powerful broadcast radio transmitters, high magnetic/electric fields, teeth both human (toddlers) and animal (cats and dogs)...
14) logistics experts to deal with shipping, returns, refunds, recalls
15) customer support
16) psychoacoustics and acoustics engineers to make sure your device doesn't sound like shit (both what you hear, and that includes safeguarding the speakers from burning out, and what others hear from you, aka the beamforming stuff that the Asahi people reverse engineered)
17) video engineers to make sure the whole darn thing isn't off color
18) camera/optics engineers, even if you acquire camera units these need to be integrated properly
19) lawyers and domain experts to deal with the compliance crap: RoHS, CE, FCC, India's regulatory authority, licensing, binary blobs, video codecs, audio codecs, carrier compliance testing, HDMI, HDCP, the RF compliance crap that's needed for US compliance [1], tariffs, sanctions laws... the list is endless
20) advertising (although admittedly, word-of-mouth could be sufficient), and PR in general (including websites, print media, AtL/BtL marketing)
21) deals with app developers, lest you end up like Windows Mobile
22) security testers/experts to make sure your devices don't get 0wned by cellebrite, mossad, nsa, cia, ...
23) human resources experts ("people engineers") to herd all the cats
You're looking at a minimum of 2-4 million $ for the engineers alone, another 4-5 million $ for the compliance crap, many millions for the app deals and way more in upfront cash for components and logistics chains.
That's why every attempt at a reasonably open source phone design has either failed or is many years behind the mass market. And the list of organisations attempting to do so include household names of the likes of Mozilla. And that is also why/how ODMs exist... they all have figured out some "minimum viable design" that gets tweaked a bit for the customer brand, and that's it. Everyone else went bust. Including, as mentioned, Microsoft. Including former powerhouses such as HTC. It's simply too complex to keep up.
On HN, we could probably drum together people of all these skillsets, no doubt (it took me half an hour to think of all these people and I'm pretty certain I've missed important aspects still!), and even ones with enough money to burn. But even then: the competition are the richest companies on the planet: Apple, Google, Samsung. Good luck...