MLL lounge membership is ~400 CAD and not worth it for one visit, although the lounges in YYZ (and in general) are excellent. The Priority Pass lounges at YYZ are in Terminal 3 (AA and KLM lounges). The OP will be in Terminal 1. MLL does not have day pass options except via lounge certificates that its own Elites can give as gifts.

Better value for $$ could be a UA RCC membership, which would give the OP access to *G lounges throughout his entire RTW, although it costs 500 USD.

YYZ T1 is brand new and not exceedingly unpleasant. For a single 5h layover you can sit in a bar or a cafe with a book and do perfectly well.