Already running OJS?
OJS is capable, free software, and many good journals run on it. If yours is well maintained and you have the people to keep it that way, keep it. Here is what changes if you move.
Someone else runs it.
No server to host, patch or upgrade, and no plugins to keep compatible. When the person who set it up moves on, nothing stops.
Discoverability is on, not installed.
Google Scholar tags, OAI-PMH harvesting, a sitemap, an RSS feed and citation export are on for every journal, with nothing to configure.
Your conference and members alongside.
PKP retired Open Conference Systems in 2023. On Fellova your conference abstracts and your membership register sit next to your journal, with one login, and accepted abstracts can go straight to the journal.
Your archive comes with you.
Your back issues come across from an Excel sheet and their PDFs, with their DOIs.
What you would give up, honestly:
OJS has a large plugin library and a worldwide community, and it can deposit DOIs with Crossref. Fellova does not register DOIs, and it is a smaller, managed system. If those matter more than the points above, OJS is the better fit.