Hi Victoria,
thx a lot for your reply. Now I don’t really need an additional gallery, but if there’s no other solution I decided to place a blank gallery at the end, just to make the first page look right (to my mind the blank space should be somehow appearing after the last gallery in the second row and not in between).

I still have some issues, which I require assistance with:
1) in the naming of one of my galleries there are 3 dots “…”, I can’t figure out where they come from. The HTML code inspection shows the following:
<div class="av-masonry-entry-content entry-content" itemprop="text">…</div>
Any idea where that might come from?
2) As suggested in one of the previous posts on the forum, I using the “post type order” to reorganize the order of the portfolio items on the front page. But something strange is to it: the order does not correspond to what is shown. If for instance I put one gallery under another inside of Portfolio > Sort, the items are not shown one after the other on the main page, but somewhere else in the same row. I have to guess now and sort the things in a way, that the appear in the order as I want them to be seen. Hope it’s clear, otherwise let me know and I provide some screenshots.
The same behavior happens inside one of the portfolios, which is being displayed – as the from portfolio overview – as the “free masonry” style. Trying to sort individual pictures inside of the gallery using the “Enfold” layout builder (i.e. inside the masonry gallery media element) doesn’t yield the expected result: the pictures are shown somewhere else, not in defined order.

Thanks a lot in advance for your help, much appreciated :-)!
Pasha