What is the best bicycle sat nav for long distance?

Just use your cellphone with a GPS app you like (e.g. Map my Ride, Waze, etc). You'll need to charge it, of course, after a day. But if you shut off extra power wasters like Bluetooth and Wi-Fi and turn off the screen it should last OK. That's what I use. Why carry two gizmos?

OldHippie - I appreciate your feedback!

So all of them require a phone whilst cycling?

I was looking at the Lezyne mega XL but it needs to be connected to a phone.

I’d be using a powerbank of course..

You're asking for something that does not exist!  After a full day of riding all GPS bike computers need to be recharged.  I don't know of a single one that would go over MAYBE a day & a half.  I use a Garmin Edge 'Touring Edition' model.  Pic blow.  After a 4 hour ride today, it was down to about 75%.  I found a refurbished one on $ALE for $85.00.  That's dirt cheap!  

The lowest priced (new) Garmin with map capabilities is about $ 249.99.  https://buy.garmin.com/en-US/US/p/611996  Even from Wally World (Walmart) the lowest priced one is $209.95.  And it states: Battery life: up to 15 hours. https://www.walmart.com/ip/Garmin-Edge-520-Plus-Bi... 

Who knows what's "Too Expensive" by your standards?  You didn't give a price.  To a millionaire, $200 is a drop in the bucket.  To me, $200 is a big hunk of change.