What do you undestand by "own-design"? are T2, A9 Apple's own-design or "own-desing", Apple didnt develop ARM architecture, just liecensed it and taylored it every year for its own products, actually Apple what does is to customize latest ARM core design for three diefferent c SOC: for iPhone, iPad, Tx mac system controller.
Sameway, licensing AMD Zen, allows Apple to customize it and name it "its own design", BS wording.
Even Intel has told some investors that they expect Apple to switch away from Intel CPUs in the near future:
https://www.tomshardware.com/news/apple-mac-arm-cpus-2020-intel,38668.html
Axios actually arent quoting Intel, no reference quoting Intel, also details switching to ARM, Intel its just aware Apple wont order anymore certain CPUs starting 2020 (neither theyre entitiled to know about Apple's plans).
Apple does not need to debug every code, iOS and macOS shares most of the same kernel,
Its evident you have never switched an kernel from platform, as example Linux Kernel took years to reach maturity to be deployed in serverts, it cost the hosting and arm millions to debug the ARM optimized kernel, not that easy.
Further, where is the evidence about Apple's testing something relted to an ARM kernel ? (t2 is not part of the Kernel, its an isolated environment, sameway Catalyst its about High level App migration from iPadOS to MacOS, no ARM-tuning implications, and Actually few Catalyst applications need some ARM and x64 code for certain in-lines not platform agnostic.
So,
now explain me, while there is no ARM-eviodence beyond biased and miss quoted articles nothing pointing to actual sources explicitly pointing out ARM? and meanwhile there are tons of evidence about APPLE testing AMD Zen platform, as removing macOS provisions to prevent runt macOS on AMD systems, to include drivers for latest and un-released AMD Apu based in Zen, Vega and Navi ?
Why?
Apple can control Mac CPU supplychain (besides other intel-related pains) just licensing AMD Zen and customizing even beyon x86-64 compatibility, to manufacture Mac-only CPUs and Mac only for this CPU.
All this cr@p about ARM based mac, it just BS from journalist and fanboys that never coded and never integrated hardware.
Only ARM-Mac with some sense possible to come in the future is an system based on iPad SOC and running Catalyst-Only applications very like Windows 10 RT systems, not replacing the x86-64 Mac Platform, but as complement for a market niche that dont need X86 performance/compatibility, but that does'nt like the iPad as Mac Replacement.