ON CONFLICT on serial id

Hello,
I have a function to perform an upsert. In order to use the id serial, if the operation is an insert, I use a coalesce.

CREATE OR REPLACE FUNCTION upsert_brand (
in_brand_id INTEGER,
in_brand_name TEXT
)
RETURNS VOID
AS $$
INSERT INTO brand (brand_id, brand_name)
VALUES(
coalesce(in_brand_id, (nextval('brand_id_seq'))),
in_brand_name
)
ON CONFLICT (brand_id)
DO
UPDATE
SET
brand_name = in_brand_name
WHERE
brand.brand_id = in_brand_id;
$$ LANGUAGE sql ;

1- Is this use case is recommended?
2- is there another way to prevent nextval ( sequence) from creating gap id if an error throw, or any trouble linked to unsafe transaction ?

Thanks in advance.
Mai