"I wonder if in this kind of wallet, ability to mark individual descriptors watchonly or not, ability to display two balances, and ability to have RPCs that know which descriptors are intended for signing regardless of whether private keys are present might help with UX, and maybe let someone get away with just having have one bitcoin wallet instead of two and having to exporting/import between them."
@Talkless have you checked out previous attempts to add this index? See for example #14053 which is still active and bitcoin core reviews session https://bitcoincore.reviews/14053.html. If you haven't seen it I think it will provide more context on the issues.
I have LOCAL full node, LOCAL electrum wallet, I would like that my (and only my) local electrum wallet would connect DIRECTLY to that local bitcoin full node via RPC and could work just like that
Even if Bitcoin Core would have a such full address index,... I think it would be naive to also add the endpoint with all the required httpd hardening and authentication methods.
We can't have that in Bitcoin-Core
I see the usecase for a personal block explorer. But IMO you should use third party software for this. There is no need to add complex indexing to the software that provides also the Bitcoin consensus protocol.
With multiwallet and enforced watch-only support, I think using Bitcoin Core as a backand for personal wallets is possible/fine.
Could it be "controversial" to implement addrindex, so that other third party wallets (like Electrum) would not need intermediary server (like Electrumx) to make it work? It could access all info via Bitcoin RPC instead.