Thanks, @mjskay. This is a very smart way to address the problem.
Still, because brms and stanarm only accept a single column of weights, only a single column of weights must be declared in the data block. Then I could use my updated Stan model via the update command as suggested by @Guido_Biele there.
In my new dataframe data_long the former values of the columns weights.w_1 to weights.w_10 - representing the distribution of weights per observation - are now linked by the same value of the variable id.
That needs to be accounted for in the data block :
data { ...
real<lower=0>; weights[N, 10]; // data block of model weights in wide format
}
where real<lower=0> weights[N, 10] needs to be converted into real<lower=0> weights_prime[k].
This k here could be a new variable defined in the dt_long representing the values of the variable weights for each value of the variable id. Not certain how to define that variable. Probably transformed data block could be the right place to do these things.
Thanks in advance for any help.