This is a real problem- which discourages especially low level players who aren't that strong yet. High level/more powerful players will always be dominant (mostly), dont think there is a work around for that. Putting level restriction will make it even harder to find convoy/plunder at times, even more on servers which are not that populated. Some people have suggested 2 key tips, and I think they should work well in your group too-

1) You get 4 chance at convoy, but 2 for plunder. So, if you wait for sometime, there is a very good chance you can do convoy without being ambushed. Doing 2 plunder takes about 10 min(if the match is balanced), 4 convoy another 10-15 min. Just wait the first half, let most people finish their convoy/plunder and then start your journey.

2) Strong players in your group, ask them to take a bit more responsibility and do multiple runs, taking not-so-strong players with them. We do this all the time in our group, and it gives good result. Watching stronger people fight, it also gives you a good opportunity to learn tactics and get better.

In general, convoy/plunder is just a basic pvp aspect of this game, same as sage field/matsuri and many other upcoming things in game. There will always, always be someone stronger than you. Don't let a defeat discourage you.