performance problem on big tables

I have performance issues with two big tables. Those tables are located on
an oracle remote database. I'm running the quert : insert into
local_postgresql_table select * from oracle_remote_table.

The first table has 45M records and its size is 23G. The import of the data
from the oracle remote database is taking 1 hour and 38 minutes. After that
I create 13 regular indexes on the table and it takes 10 minutes per table
->2 hours and 10 minutes in total.

The second table has 29M records and its size is 26G. The import of the
data from the oracle remote database is taking 2 hours and 30 minutes. The
creation of the indexes takes 1 hours and 30 minutes (some are indexes on
one column and the creation takes 5 min and some are indexes on multiples
column and it takes 11 min.

Those operation are very problematic for me and I'm searching for a
solution to improve the performance. The parameters I assigned :

min_parallel_relation_size = 200MB
max_parallel_workers_per_gather = 5
max_worker_processes = 8
effective_cache_size = 2500MB
work_mem = 16MB
maintenance_work_mem = 1500MB
shared_buffers = 2000MB
RAM : 5G
CPU CORES : 8

*-I tried running select count(*) from table in oracle and in postgresql
the running time is almost equal.*

*-Before importing the data I drop the indexes and the constraints.*

*-I tried to copy a 23G file from the oracle server to the postgresql
server and it took me 12 minutes.*

Please advice how can I continue ? How can I improve something in this
operation ?