Example of partial aggregation

sum values for each type (eg, for type=1 - 1+2=3 and for type=2 - 3+4=7) and then combine it with by for example multiplying the subset results (eg, 37=21).

select my_multiply_agg(sum_per_type)
from(select sum(value) as sum_per_type
     from my_table
     group by type) as subquery;

CREATE OR REPLACE FUNCTION my_multiply_agg_sfunc(state numeric, value bigint) 
RETURNS numeric RETURN state  value;
CREATE OR REPLACE FUNCTION mymultiplyaggcombinefunc(state1 numeric, state2 numeric)
RETURNS numeric STRICT RETURN state1 * state2;
CREATE AGGREGATE mymultiplyagg(bigint) (
    SFUNC = mymultiplyaggsfunc,
    STYPE = numeric,
    INITCOND = '1',
    COMBINEFUNC = mymultiplyaggcombinefunc,
    PARALLEL = SAFE
);

CREATE OR REPLACE FUNCTION mysumsfunc(state bigint, value integer) 
RETURNS bigint RETURN coalesce(state,0) + value;
CREATE OR REPLACE FUNCTION mysumcombinefunc(state1 bigint, state2 bigint)
RETURNS bigint STRICT RETURN state1*state2;
CREATE AGGREGATE mysum(integer) (
    SFUNC = mysumsfunc,
    STYPE = bigint,
    --initcond replaced by coalesce() in sfunc and making combinefunc strict
    COMBINEFUNC = mysumcombinefunc,
    PARALLEL = SAFE
);

After "fixing" that, you'll find your function is nondeterministic. The order of + and is significant: 23+4 =10 but 2+3*4 =14. Since there's no way to guarantee which parallel worker will pick which subset of rows, same input can lead to different outputs at different times.

Another thing to keep in mind is that regular sum() of int returns bigint, otherwise it'd risk overflow. Since you end up multiplying sums, that's even more of a concern in your case. Above bigint, you'll need numeric, which is what I switched to, in the example up top.

These are logical problems but functionally, your functions are fine. As immediately pointed out by @Bergi, confirmed by @Laurenz Albe and demonstrated on dbfiddle, you just need more rows to trigger parallel execution. Here's your example on 300k:

explain analyze
SELECT mysum(v.value)
FROM mytable v;

SELECT mysum(v.value)
FROM mytable v;