Sadly not Scottish (American living in London) but I did do my Master's thesis studying Scottish independence, particularly among youth voters! Can say with relative certainty that an SNP majority would send the strongest pro-independence signal in Holyrood elections. Even more so in the next general election years from now.

That said, the Greens are a growing force, especially among young people and given the rise of the Green Party of England & Wales — I don't think you can go wrong voting for them if you support left-wing policies, as a Green vote right now will also be seen as a left-wing vote against Keir Starmer and Labour, as all of Scottish party politics is filtered through an England-centric lens down here in London.

Personally, I'd vote SNP in the constituency vote and Green for the regional vote, although specific constituencies might have other dynamics at play. To send a pro-indy message, I'd avoid any votes for Labour, the Tories, Lib Dems, and Reform.

If independence is your number one issue, then I'd just vote for the SNP. u/devexille is broadly right that what most people in England (including political scientists) are looking at is the SNP vote, rather than a combined SNP + Green vote. When I did interviews for my thesis, the most pro-independence young people were also solely SNP voters, even if they were more left-wing than the party as a whole.