There’s no easy answer here, something has to give.
What is likely to happen is your app spending will decrease somewhat as percentage of the Confluence/Jira host cost as you migrate from server to cloud, especially if you have a lot of apps installed.
In terms of the apps you mention (which I’m guessing are your largest increases ), take Scriptrunner. Jira Cloud does have automation functionality built-in. It’s worth seeing whether you can replace Scriptrunner with the available functionality.
If you can’t, try to see if the Jira automation roadmap has plans for the features you’re lacking. If yes, there’s the option to wait for them to become available and migrate at that point.
In terms of Gliffy, the obvious answer is draw.io. draw’s pricing is much more reasonable and has a stronger focus on things like security, as well as collaborative editing, which Gliffy lacks.
The standard (non-Enterprise) version of draw for Confluence Cloud costs 4,525 USD per year for 2.5k users. Your Gliffy cost actually more than halves.
draw.io has a one-click migration tool to migrate all Gliffy diagrams in one operation.
Just note that Gliffy doesn’t actually migrate correctly from server to cloud. One particular problem is it doesn’t update pageIDs in links in diagrams pointing at pages. If you have thousands of diagrams manually updating them all would be a massive task.
draw.io does support migrating pageID between instances, so if you decide you want to migrate it’s important to migrate to draw.io on your server instance, then use the draw.io migration process to move to cloud.