on the search make it possible to show only completed but decide wheatther to show oneshots, series, or both

This can already be done using Advanced Search by Genre.

This is what is there:
Show all manga
Only show completely scanlated manga
Only show completed (including oneshots) manga
Only show one shots
Exclude one shots
Only show manga with at least one release
Only show manga with no releases

This is what im suggesting:
Show all manga
Only show completely scanlated manga
Only show completed (excluding oneshots) manga
Only show completed one shots
Only show one shots
Exclude one shots
Only show manga with at least one release
Only show manga with no releases

and is there no advance search bottun? Cause i always have to search a random manga and click on the tags to get to search by genre

go to "genres" in the box with "manga search" above it, then click on "advanced search by genre" near the top.