Try this.

https://brouter.de/brouter-web/

Click once to set start point, click again to add a destination, repeat as necessary. Drag an existing line to adjust. Drop down menu at the top to select different flavors of routing suggestions. Export to GPX file and open the GPX with the Garmin app to upload to the GPS. All of the above done on the phone.

Or www.cycle.travel, which can connect directly to Garmin.