Who did you manage to get an agreed policy from without the usually
classic/collector car restrictions?  It seems everyone I have talked to will
not give an agreed amount on a regular use policy (drive whenever, wherever)
or will always pop in driving restrictions such as not to work, regular use,
and so on.

There really is a need for that middle ground.  This is being discussed on
CF right now, too.  A regular use policy that allows you to declare an
agreed to value.

Realistically, it would be higher than a regular daily driver policy, but
not that much.  The risk is the same, but instead of paying out on a $30,000
car only $25,000, they would still pay the full $30,000.  Actually, when a
car is new, they would be paying out the full amount or nearly that, so it
isn't like this is an outrageous price, unless they have averaged the payout
price over a few years ($30,000 when new, $25,000 when 1 year, $20,000 when
2 years, $15,000 when 3 years, and so on).  Since they could be stuck when a
new car is totaled, I seriously doubt they do that. They are probably
insuring against the highest full loss price, and just never readjust/lower
for the current value.  So really, insuring for a $30,000 agreed value
should be within a few dollars of insuring a $30,000 regular driver at the
start.

I've found insurance to never be a linear amount anyway.  You might pay
$100/6 mo for $20,000 liability. But increasing , you might pay $150/6 for
$100,000.  And maybe $170/6 for $300,000.  Doubling or tripling the coverage
does not double or triple the premiums.