Как условно да групирате в колона, без да използвате FULL OUTER JOIN

Това е просто условно агрегиране, доколкото мога да преценя:

select id, 
       array_agg(amount) filter (where type = 'Customer') as customer_array,
       sum(amount) filter (where type = 'Customer') as customer_sum,
       array_agg(amount) filter (where type = 'Partner') as partner_array,
       sum(amount) filter (where type = 'Partner') as partner_sum
from table_a
group by id;

Ако искате празен масив вместо NULL стойност, обвийте функциите за агрегиране в coalesce() :

select id, 
       coalesce((array_agg(amount) filter (where type = 'Customer')),'{}') as customer_array,
       coalesce((sum(amount) filter (where type = 'Customer')),0) as customer_sum,
       coalesce((array_agg(amount) filter (where type = 'Partner')),'{}') as partner_array,
       coalesce((sum(amount) filter (where type = 'Partner')),0) as partner_sum
from table_a
group by id;