Litigation E-Mail Tool for Collection / Analysis?

I've been tasked with doing some initial discovery on around 40 email boxes that are split between Gmail and Microsoft 365 / Exchange. I'd like to get advice on a practical tool we can use to download and search the collection of mailboxes, allowing us to use keywords and constraints to find e-mail and then collect them into a PST or other repository for delivery to our lawyers.

Any advice?

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