Great! 
thank you all for the advice, i'll go with the last solution, a new sqlserver 2016 AG box with log shipping from 2014 box

Thanks for the reply
unfortunately the procedure is for an update in place
I have a 20TB db that I have to migrate into another cluster that doesn't share the...

it's me again...

another test, I would like to go deeper on this issue
same database, but with 13K + filegroups/datafile
now it's running and stuck in "fixups" phase.

hi,
I would like to give you an update about the restore tests. I restored the smallest one,7TB with 4600 files.
It took a lot less time. the high...

select from sysaltfile blocked too  🙂
session_id,wait_type,sql_text,blocked_by
52    LCK_M_S    select * from sys.sysaltfil    55
55    NULL           RESTORE DATABASE [dbxxxx] FROM DISK...  0

just launched the restore of the db with two commands, the first for full restore with NORECOVERY the second with diff and RECOVERY option
No buffers related options added

Db restored on the test server, checkdb has found the same errors as expected
I did things suggested by Gail, (drop-create the index / tables), after this, the chekdb has...