Jet engines are indeed hard. But with the upcoming tech transfer from Ukraine as well as the slow but steady improvement of the WS-15, I would be incredibly surprised if it took more than 6 years for the WS-15 program to yield a serviceable engine.

Even the US relied on foreign jet engines for a time, although you won't hear about it a lot - for example, an absolutely huge amount of engineers were hired from AVRO in Canada after the development of the superior Iroquois engine (which had by quite a margin the highest nominal thrust as well as the highest) as well as other Orenda engines, and their work was absolutely instrumental to the US becoming superior to the USSR in jet engine technology. I don't know if this is publicly available information, so you might have to trust me on it, but I guarantee you even the US had a 20+ year period where it had to steal engineers and tech in order to jumpstart it's lead, which is exactly what we are seeing with China.

If the WS-15 ends up being serviceable in 6 more years, it will have taken SARI and Xi'an almost 35 years to re-create something marginally better than the current Russian engines, and no where near what the US is delivered routinely by PW and GE. Chinese engines can't approach the thrust of Western engines, nor the reliability.

So say they iron out the kinks in the WS-15 (though odds are they'll settle for the WS-10). They then have a roughly 1990s era engine, when the US will be deploying Adaptive Cycle Engines etc.

Sure, but this always how it works. The first high-quality engine will take 35 years, the next 15. The important thing here is to develop the capability to manufacture high-quality single crystal components.

As far as adaptive cycle engines, China is also already researching them. They also have a lead in supersonic combustion as of today.