For this task we can collect all children for every account and then sum nominals to parent account.

1. Anchor part of recursive query.
   We take all accounts from table as root.

2. Recursive part.
   Recursive collect children’s (thru link to parent), children’s of children and so on. Column rootAccount is the same for all children on every branch of the tree.
   Output of recursive query below.

3. Aggregate nominal by rootAccount.

Here lvl and accs columns – for debug and clarity.
See example

Your source data

Recursive query

with r as(
  select 0 lvl,AccountSeq rootAccount,AccountSeq incAccount,Nominal
  from tAccount
  union all
  select lvl+1 lvl,r.rootAccount,t.AccountSeq incAccount,t.Nominal
  from r inner join tAccount t on t.ParentAccountSeq=r.incAccount
)
select a.*,t.sumNominal,accs
from tAccount a
left join(
   select rootAccount,sum(nominal)sumNominal
     ,string_agg(cast(incAccount as varchar),',') accs
   from r
   group by rootAccount
  ) t on a.AccountSeq=t.rootAccount

Details.
Recursive query output before aggregate

with r as(
  select 0 lvl,AccountSeq rootAccount,AccountSeq incAccount,Nominal
    ,cast(AccountSeq as varchar(100)) path
    ,AccountName
  from tAccount
  union all
  select lvl+1 lvl,r.rootAccount,t.AccountSeq incAccount,t.Nominal
    ,cast(concat(path,',',cast(t.AccountSeq as varchar(100)))as varchar(100)) path
    ,t.AccountName
  from r inner join tAccount t on t.ParentAccountSeq=r.incAccount
)
select * from r order by rootAccount,lvl;

fiddle

Reverse the recursion and group by accs to get hierarchy of totals

WITH cteTest AS (
    SELECT
    AccountSeq,
    ParentAccountSeq,
    AccountCode,
    AccountName,
    CAST(Nominal AS DECIMAL(18,2)) AS Nominal
    FROM
    [Account] a
    WHERE not exists (select 1 from [Account] b where a.AccountSeq =b.ParentAccountSeq)

    UNION ALL

    SELECT
    a.AccountSeq,
    a.ParentAccountSeq,
    a.AccountCode,
    a.AccountName,
    cte.Nominal
    FROM
    [Account] a
    INNER JOIN [cteTest] cte ON cte.ParentAccountSeq = a.AccountSeq
)
SELECT AccountSeq,
    ParentAccountSeq,
    AccountCode,
    AccountName,
    sum(Nominal) nom
FROM
[cteTest]
GROUP BY
    AccountSeq,
    ParentAccountSeq,
    AccountCode,
    AccountName
ORDER BY 
    AccountSeq