Migrating custom pages to SharePoint online

Hello! My company is finally migrating from SharePoint 2016 to SharePoint Online. Very exciting and very frustrating at the same time. We have a lot of custom pages in SharePoint and we just learned that the classic mode won't be an option. From what I understand, modern experience doesn't allow custom JS/CSS on a page. I would love to to learn what alternative solutions those who migrated found to solve this problem.