Level Up (A5E) Attacking with a shield uses improvised weapon proficiency

[Tessarael]

Doffing and donning a shield requiring an action, per LUA5E page 320, does greatly lower the effectiveness of throwing it. And you cannot have a shield that is both hands-free and has the rebounding property, as it explicitly specifies that hands-free shields cannot be thrown on LUA5E page 324.

Tower shield allows you to plant as a free object interaction and unplant it as a bonus action, per LUA5E page 317, which presumably doesn't require you to don it again. At least that's how our campaign has been running it.

There are a few ways of speeding up doffing and donning armor such as the Fighter Knight archetypes armory training or using a Squire, but I don't see anything that reduces donning a shield below an action.

 

[Tessarael]

There's nothing in the rules that says you couldn't train a particular type of improvised weapon, e.g., a shield or wooden stake, as a rare weapon. I don't think it would be unreasonable to allow that.