Thanks a lot for the advice!
I’ve switched most of the sections into a row/column structure, and it seems to keep the alignment on all screen sizes as far as I can see now.
I set the width of the main picture back to 100% within the white box, as I didn’t like so much white space aesthetically, but by aligning the box itself to the center, it seems to still work at all resolutions.
The only slight issue I have now is that I want to align the final text (with the link) to the bottom of the screen (at all resolutions) and can’t seem to manage to do that.
Anyhow, thanks a lot for your help!