Site Hacked Recovery Process

My site has been hacked back in January, I recovered it from a backup and I began to build more links. The problem is that those hackers created more than 300,000 pages that redirected to their spammy Japanese sites, in GSC Google flagged some of them as 404, but many of them are still in the index and my rankings didn't come back. Do you suggest to do a 301 redirect and move the content to another site? Google didn't flag my site as hacked in GSC and I didn't receive any manual penalty.

Are these 300k pages or just URLs? I've had the same and by getting the URL removed via GSC (temp removal for 6 months) I've managed to go from 8k URLs to 144 (which is correct for my site).

I believe there is a recommended process for removing URLs from the index in these cases. Follow the guide here -

https://support.google.com/webmasters/answer/9689846?hl=en
If you follow the required steps then you should be able to get things back to full working order.

For the record, if you are concerned about going forward, about the safety of your website in terms of hacking attempts, I recommend Malcare. I use this and have had excellent results so far across multiple sites.