I added following lines in the build-in custom filters:
youtube.com##ytd-rich-grid-row, #contents.ytd-rich-grid-row:style(display:contents !important;)
youtube.com##ytd-rich-grid-renderer:style(--ytd-rich-grid-items-per-row: 6 !important;)

Youtube page should display 6 columns in a row as follows:

Instead it just show 3 columns.
The lines work in uBlock origin custom filters:

0.99.130.2-beta
Sequoia (15)