How can I change replication slot's restart_lsn from SQL?

Hi,
I'm working on a workaround for a bug in Pg
(https://www.postgresql.org/message-id/flat/Yz2hivgyjS1RfMKs%40depesz.com)
I want to create replication slot, and advance is manually, keeping it
always a bit lagging behind real replication slot.

I can create slot, no problem:
select pg_create_logical_replication_slot('depesz', 'test_decoding');
and then I can, theoretically, advance it to whatever position with
select * from pg_replication_slot_advance('depesz', '...');

*BUT* - it changes only confirmed_flush_lsn, leaving restart_lsn as it
was.

How can I advance restart_lsn of a slot?

Generally my idea is to get lsn from real replication slot, subtract,
let's say 1GB from it, and advance my "fake slot" to this value, this
keeping always buffer of 1GB in case the bug with removed wal happened
again.

I will be doing this on Pg12, which can limit my options, but perhaps
there is some way to do it via plain(ish) SQL ?

Best regards,

depesz