Documentation is really lacking in this regard, that's a shame. After reading tons of stuff, that's what I came up, in Angular 7:
Removed the npm package fusioncharts, but kept "angular-fusioncharts": "^3.0.1"
Created a folder libs in the root of my project, then put inside this folder the content downloaded from the link in the purchase email, renaming to fusioncharts
Changed my imports from:
import * as FusionCharts from 'fusioncharts'
import * as Charts from 'fusioncharts/fusioncharts.charts'
import * as PowerCharts from 'fusioncharts/fusioncharts.powercharts'
import * as FusionTheme from 'fusioncharts/themes/fusioncharts.theme.fusion'
to
import * as FusionCharts from 'libs/fusioncharts/js/fusioncharts'
import * as Charts from 'libs/fusioncharts/js/fusioncharts.charts'
import * as PowerCharts from 'libs/fusioncharts/js/fusioncharts.powercharts'
import FusionTheme from 'libs/fusioncharts/js/themes/fusioncharts.theme.fusion'
The init code was the same:
FusionChartsModule.fcRoot(FusionCharts, Charts, PowerCharts, FusionTheme)
I don't understand why I was getting an error if I kept the import * as FusionTheme for the theme package, I had to change to import FusionTheme from.
I put "allowSyntheticDefaultImports": true in my tsconfig.json at some point during this process, not sure if this configuration is needed for this last step.
Some (not that) useful links:
https://www.fusioncharts.com/dev/getting-started/angular/angular/configure-your-chart-using-angular
https://www.fusioncharts.com/dev/upgrading/upgrading-from-evaluation-versions