I have only just spotted this thread and see no one responded to your question when it was posted.
So the short answer is Yes if you have enough patience and small fingers you can indeed create a custom route direct on your satnav or mobile device. However it is probably a lot easier to create the exact route your want to take on a large screen device.
Meantime up the Alps we are already suffering from lockdown fever and are busy devising possible routes for tours we hope to lead next summer provided this virus is no longer an issue.
Typically we need a way to reliably and easily share the planned daily route (Monday Morning/Monday Afternoon etc..) with other riders who doubtless will have an unpredictable mix of mobiles and TomTom and Garmin kit.
Hence we use ITN Converter ( a free/shareware app) as a key intermediary between the route URL which we always devise in Google maps and whatever format other riders need.
OK anyone planning to use a google device can simply run with a google maps URL direct but others will need the route in their format.
So basically create the route using the Google maps idea fully described earlier in this thread and once it is right, simply right click and choose copy on the google map URL at the top of the browser page - (it is usually very long)
Then create an email to send to the other riders with separate URL links for each ride.
so Monday morning ride:-www.google
Monday afternoon ride:-www.google
Nowadays most riders prefer to download the daily route plan (which is great if groups get split up or someone gets lost or has a breakdown and then needs to catch up later).
The process is simple :
(1) download a copy of the ITN Converter program. (Make a donation if you wish)
(2) launch the program and do a very basic set up (only needed once) to choose your preferred output format - (garmin, tomtom ...) and preferred drive/folder destination.
(3) paste the relevant daily ride link into ITN Converter which will spot it is a web URL and ask permission to connect to the web and download the entire link waypoints list - which will swiftly appear on screen.
(4) click export to convert the google maps URL into whatever satnav format you have selected during set up.
The file(s) can then be transferred via a suitable SD/micro SD card and installed in your GPS or sent by cable to your GPS.
On a TomTom the files can then be found and selected from the Itinerary Planning link (other GPS makes even LIDL units will usually have something similar).