OTOH, if you're a medium-sized country with a reasonably sized fleet and can't induct the J-35, F-35, Su-57, or Kaan, then the KF-21 is your best option for properly gearing up for modern air warfare. Its radar isn't as competitive as those found on those aircraft, but it still has an IRST and can hold its own against enemy non-LO platforms. In a clean configuration, it can achieve a semi-LO signature, you can train against adversary LO fighters with it and develop a CONOPS, and it'll also do just fine against virtually any other medium-sized country (except European ones, TR/SK/JP/PK/IN, or ISR despite its small size).
I very much doubt KF-21 signature is substantially different from 4.5 gens or hotspot-treated 4th gens.
A 0.5m2 jets appear to take way more to achieve than what's done here (pre-1982 ATF designs as a reference point); a 0.1 one(su-57) is in a different universe.
It logically leaves it in the same 0.7-1.5 group.
It may be stealthier/unidentifiable via blade counting, but other than that - it should be in the same general same area; 0.1m left or right in this case does very little to the eventual result. (halving the RCS reduces detection range by just 16%).
KF-21 advantage in the detection game thus isn't really signature - it's the powerful and advanced GaN radar. Which is nice, but by itself it is not a generational leap in fighting capability, unless it's a MESA (which doesn't appear to be the case).
I.e. a truly low observable KF21 will wait for blk.3 (if ever). Until that, with it's weird MRAAM comb taking 1/3 of useful real estate, KF-21 just struggles to offer flexibility with any configuration that doesn't need 4 MRAAM. Granted, it should be great and superior when you do need them; but how often do you need 4-6 AAMs? And if your main sell is hotrod "medium a2a fighter" with some strike - then you're stuck being mig-29/eurofighter.
With all the Viper problems, its risks are very few systems, which should close one way or another before any deliveries signed today will materialize. The rest is solid existing/developing US weapon ecosystem.
Or, two ecosystems, as the Israeli remains fully available as well, and it's unfortunate - Israel sells all the same weapon options Korea wants to in the future, just proven.
In KF-21 case, the risk is almost everything (nothing of it's weapon ecosystem is born to date, i.e. non-risky are meteor/aim-120/iris-t). More annoyingly, US already played let you/won't let you integrate game with Korea (FA-50 - AMRAAM, JASSM order back in the day).