Hello,
Just as an update that it worked like a charm:
1. I have built well my finances through Enterprises and Land Ownership.
2. Purchased a ship and docked it on the
port of Sargoth (siege = ladders), filling it with 2,500 troops recruited from everywhere.
3. When the right time has come (Nords just declared war on Vaegirs), I have purchased 5 other ships and docked them at the
coast (not the port) of Sargoth.
4. Started transferring troops from Sargoth to these 5 other ships until the one docked on Sargoth had no troops left.
5. Stormed the town with elite troops and, after conquering it, quickly tramsferred all the troops from the 5 ships into it.
I ended up with a town containing a garrison of 2,500 troops in less than a day
NOTE: The reason I choose to dock 5 ships instead of 1 is that, even when properly guarded, something bad might happen to the ships when on the coast and not a port (the mod doesn't explain, just mentions that you had a "bad feeling" about the ship and it just vanishes with all your troops from it). Hence, it's best to split your troops in 5 (or any other number that makes sense), dock them right outside the town to not lose sight of them and quickly attack it. If you end up losing 1 ship (didn't happen with me fortunately), it's only 1/5 of your troops.
For roleplaying, I imagined my troops disguised as fishermen.