The fast train to Mestre and then changing to the regionale for Treviso is pretty straightforward. You may have to walk down the stairs and cross underneath to a different track and walk up to that track but that is all. Most of the trains that day have a 20 minute wait will be long enough to get bored. If you have the Trenitalia app you can even check the track for the next train before you get off so you'll know where you're going ahead of time.
Trenitalia has 2:45 - 3:15 fast train plus regionale options for about 26e - if you buy now - which seems pretty reasonable. The super cheap fare in economy on .Italo is lower at 18e but their next tier is also 26e so it's about the same if you miss the 'magic' pricing.
Pricing of fast trains increase as you get closer as discounted tickets get bought up and you get closer to the 'base' or full fare being the only option day of. If you're willing to trade flexibility for price I'd buy as soon as you're ready to commit.
Changing trains in smaller stations is a non-event that people worry about because it's not something we generally do, but this is just everyday life in Italy. Get the app.
Have a great trip,
=Tod