Don't get disheartened!

It really is not that difficult!
Get a Nordic tollbox from
tolltickets.com (a German operator). A one time fee of € 5.90. That box covers Denmark, Sweden and Norway, including the Danish bridges, with a discount of 10-20% on road tolls and a 10% discount on Norway local ferries included, without further ado. Works like a credit card: you get an invoice from the tollbox operator.
If you want a 50% discount on local ferries in Norway, too, you have to set up an account with
autopassferje.no.
- Register your tollbox there.
- Make a prepayment of NOK 2200.
Done!
Their system works like a debit card: autopassferje charges your account with them for the ferry trips. Any unused money you can claim back: it comes with 2-3 days back to your bank account. As soon as the balance on your account drops to NOK 0 or below, you either top it up or the system refers back to the credit card system of your tollbox: you get only a 10% discount, and are invoiced from the tollbox operator, for as long as your balance is below 0.
Attached is a list of Norway ferries in the system. The autopassferje thing has saved us a few hundred Euro, each trip

!
There is one caveat, though. When I say it's easy, that is from my EU-citizen point of view. International (re-)payments, international deliveries, are easy and quick within the EU and the associated countries such as Norway. It may be more costly and less straightforward for UK-citizens since Brexit, I don't know.