Something like when hovering over the garrisoned unit icon. Showing the unit experience doesn't mean much, as all I need to know is if the unit reached the desired rank or not. On single unload, send out the units with the highest rank first.

If we accept the rock art is sufficient enough evidence, we must at least accept it is a minority of the boats. Most don't have sails.

Could we have a big minimap? Like in Nani's mods.

Well the main way units gain experience is by having a successful fight, using healers, or moving wounded units to the back of the group. I think it is a desirable outcome that  units generally take a long time to rank up by garrisoning, but it would be nice to show the mean rank of the units on the corresponding icon of the unit garrison stack within the barracks.

Honestly the current scout ship is not too dissimilar to the nydam boat, except for the sail. Maybe we could turn it into the merchant ship by removing the shields and adding a side rudder and more cargo?

Ok, I got in touch with Dr. Bengtsson, who authored some of the works we discussed here and she gave some helpful feedback. The Hjortspring boat was paddled, not rowed, but rowed vessels likely existed as well. I think its ok to keep the oars since we would have floating paddles otherwise. radially split logs became used in the same time frame as the hjortspring boat is dated, and were used to build the nydam boats. so I think we could make use of a nydam style boat for some visual variety. Sails were used, likely square shaped and made out of varying materials. if sails are used on a boat, either a side rudder or double-steering oars should be used as well. Single masts are supported by rock art and stone ship monuments, but bipod masts are also possible. Perhaps if we do more of a nydam style boat (merchant ship and maybe arrow ship), it could use a single square sail with a side rudder I personally think something like the Himmestadlund-1a could be a beautiful inspiration. Rock art and monuments suggest platforms at the stern like what @Lopess has on the hjortspring model. outriggers are not used outside of the pacific, so no boat should have an outrigger.