Hi Leadforms,
Interesting question.
Having lots of redirects will slow down your site.
There are different ways to implement your redirects other than .htaccess but I believe this would be the best method for you.
Is there any way you can group some of the redirects? maybe part of the URL is the same for all the pages you want to send to a certain folder then the match may work?
If not then I think you have a massive job on to redirect 5k pages and manage the redirects afterwards as well.
I would look at the pages and judge the value of a redirect on that page. If the page has no external links pointing to it and it has a low PA then do you need to redirect it?
You also need to keep in mind a few other things:
1. You need to make sure that the page you are redirecting to and from are similar in a boolean context - that is that a machine will see them as the same content and not just a person
2. you need to avoid redirect chains - that is a redirect to a redirect and so on
3. If you have redirects in place at the moment from HTTP to https you need to redirect these separately otherwise you will generate a chain
4. I would amend all your internal links rather than relying on the redirect otherwise again you will create a chain
Once you have implemented the redirects you will need to monitor them to see how much traffic they are generating, if after a few months a redirect is generating no traffic and all the traffic is going to the new URL then you will need to consider the benefit of keeping that redirect in place or removing it to improve site load times.
I hope this helps,
Steve