Fridge / Star Trek: Picard S3E07 "Dominion"

• The reveal that Tuvok has been replaced by the Founders makes sense on a strategic level. Tuvok was Janeway's closest friend and ally aboard Voyager (at least initially before Chakotay made inroads). Compromising Tuvok would remove that pillar of support and isolate Janeway even further — especially as, given he's a Senior Captain and former Tactical Officer, Tuvok (like Ro) might very well have spotted the organization-wide incongruities and realized something was very wrong in Starfleet. "Tuvok" was also likely being truthful when he said that he and Janeway had not spoken for some time. Nailing a Vulcan's character is hard and Janeway's known Tuvok for decades; if anyone could realize something was off with the Vulcan, it'd be Janeway. The logistics of overseeing Frontier Day would thus provide the perfect in-universe excuse for Tuvok to be "unable" to speak with his old Captain and avoid triggering suspicions.
• It also makes sense both in-universe and on a Meta level that of all the Voyager veterans outside of Janeway, Seven would seek out Tuvok to try and reach Janeway (rather than Chakotay). The former Captain and her Tactical Officer were old, dear friends even before Voyager. Tuvok, as a centenarian Starfleet veteran, would also have contacts throughout the organization that they could trust and use against the conspiracy. On a Meta level, Picard also couldn't use Chakotay in any capacity or spoil his fate on Prodigy (as the show's still ongoing); Tuvok was a logical compromise. Given how difficult Vulcans are to nail, they may have also thought Tuvok was less likely to be a Changeling candidate compared to Chakotay — and thus would be worth the risk of contacting him.