With Cryptomatte Tools addon there is no need to manually pick objects in Compositor and it is possible to pick objects or materials even if they are not visible in current frame. The addon also keeps track of objects and materials in Cryptomatte nodes, and if you change object/material name, it will automatically update the Cryptomatte ID in all Cryptomatte nodes where it is used. It also offers automatic update of Render Result even if the Compositor is not open when adding or removing objects/materials from the active Cryptomatte node. And it is possible to append all objects from the active Cryptomatte node to selection (if selection is empty, it is a quick and easy way to see what objects are chosen in the Cryptomatte node).
Please watch the video to see it in action and how to use it.
If you are looking to automate things, Cryptomatte Tools implements Python API to get Cryptomatte IDs, add them to Cryptomatte nodes and do many other useful things (this video does not demonstrate Python API).
Currently this addon is a beta so please report any bugs or performance issues if you find them. In final version (I plan to release it in 2021 Q1) there are maybe some changes in Python API and other improvements.
GUI screenshots and Python API documentation: https://blendermarket.com/products/cryptomatte-tools/docs
The addon can also help render Color ID maps for objects or materials.