[Refactor] replace querySelector with getElementById where possible, cache querySelector calls where posssible and needed

[jxn-30]

DOM-Operations and querySelector are heavy operations. getElementById is much faster than querySelector('#id'), especially if called often (e.g. in loops and intervals).
If multiple operations need to be done on an element, avoid selecting the element multiple times but cache it (store it in a variable or in an object) instead.

ESLint Rule unicorn/prefer-query-selector needs to be explicitely be disabled therefore.

An ESLint Rule enforcing us to prefer getElementById wherever possible would be good!

[jxn-30]

#3162 introduced replacement of querySelector calls with getElementById where possible.
Next step: Check if any querySelector call can be cached.