Right now, these two game modes are essentially Battle Royale modes. The only times I see either game mode end with a captured base is if the last enemy runs off, the base is capped before the last few enemies are picked off, or one team throws all their tanks into the cap and win in the first five minutes.
None of these scenarios are conducive to gameplay and there's not even really an incentive to approach the cap. Most of the time players will just move to the flanks that they usually do.
Both game modes are also extremely poorly balanced because the maps are not designed for the game mode. Encounter on Siegfried Line is stupid because the cap is in the middle of the town in the open.
There needs to be a way to make Encounter feel more like a battle for the cap. Assault needs to feel more like… Well, an assault.
My thought is to bring in elements from Frontline. For Assault, do away with the base. Now your objective is to destroy an enemy bunker, which can't be damaged by arty or penned from the front.
I don't really have an idea for a solution to Encounter. Perhaps having a tank in the Encounter cap could only slow down the cap by one tank. So if one enemy and one ally are in the cap, nothing happens, but if one enemy and two allies are in the cap, you earn points at the rate of one tank being in the cap. The base capture circle could also be larger.