I am currently introducing attributes on my portal.

The function is missing here:

Searches for digital attributes, between "search limits". For example, the year of production of the car, shuld allow user to search for a choice between, for example, 2003 - 2008.

the same for one date, eg 21.04.2018 and be able to find this date among other dates. for valid one day offers, eg I go (date) to Belgium, someone wants to go?

Unnecessarily all attributes show in /search page without going to a specific category. If you do not check "add to search", you will not see attributes at all in the subcategory.

A lot of attributes in many categories = mess. It would be good to group in addition.

In categories below level 2, the attributes do not show. eg in the category motorcycles / accessories / helmets = no possibility to set attributes for the helmets only.

If many checboxes in attribute, shuld be option hide checboxes after eg 5 checboxes in attribute and dropdown the rest.

You know it's possible to do such search, but consider how many variations there could be? less than, higher than, between, outside, equal....

To disable fields for no category, in functions.php find:
function atr_search_form($catId = '') {
  $cat_id = isset($catId[0]) ? $catId[0] : '';

bellow this line add:
  if($cat_id <= 0) { return false; }

A lot of attributes in many categories = mess. It would be good to group in addition.

this is a very good idea!!

I agree as well

Searches for digital attributes, between "search limits". For example, the year of production of the car, shuld allow user to search for a choice between, for example, 2003 - 2008.