Little doubt the Liaoning and Shandong have decades of service still left. But I think they could train for future STOBAR carriers in the PLAN's battle line and leave the main helo and tilt-rotor role to the LHDs.
STOBAR afford better load for fixed-wings than STOVL. Judging from the programs of UK, Italy, S. Korea and Turkey, ramped carrier are still very viable. Especially when you are using it for fighters with A2A loadout for air coverage.
Again, China does not have foreign bases in the West Pac island chains. PLAN might want to have a tiered-carrier system of CATOBAR CV/CVNs, STOBARs and catapult-equipped LHAs to saturate green and blue waters.
I don't see a future where the PLAN will procure any additional STOBAR carriers in the future.
I don't think we need to think too hard about what CV-16 and CV-17 will do in the future when they have a larger fleet of CATOBAR carriers and 076s.
We know J-35s and J-15Ts are backwards compatible on CV-16/17 despite being primarily intended for CATOBAR carriers. They'll likely continue to serve as regular, fully combat capable carriers, but just less capable than the proper CATOBAR CVNs and CVs of the future.
The idea of CV-16/17 being relegated to training role is also unlikely.
The best comparison is to think about older destroyers like 052s, 051B, 052B, Sovremenny etc -- despite being far less capable than modern 052Ds and 055s, they remain as combat capable surface combatants, rather than being training vessels. They're just roundly accepted as being "second rate" and that's fine. For CV-16/17, so long as they remain active, I expect them to be retained as regular in service carriers.
Dedicated training carriers don't make much sense anyway. The training would be done with regular in service carriers of all types as part of their training/operations cycle