What is the alternative for an exchange which doesnt want to run a node of a coin traded? It seems to be best practice for
an exchange to run a node itself for a
coin traded there.
But, is there an alternative?
A third party service? If yes, which are recommended and why?
It seems to be best practice for an exchange to run a node itself for a coin traded there. But, is there an alternative?

Alternative is to rely on someone else as mentioned by sipa which isn't best practice although few exchanges do it and face consequences for it.
Example: https://www.reddit.com/r/Bitcoin/comments/hpg9j3/336_btc_hacked_from_cashaa_they_were_using/fxrrbiw/

A third party service? If yes, which are recommended and why?

Lot of third parties provide services that exchanges don't have to run their bitcoin full node. I would never use an exchange if I know they are not competent enough to run full node and manage users funds securely.
Also service recommendations are offtopic here: How should questions that function like reviews be handled?