Could I sometimes indicate completed actions using the passive version of the present tense?

The use of the present tense varies, and thus may be responsible for the confusion you are facing.

When I say that the "work is done", the word is serves as a linking verb between work, and done. done is merely an adjective for work, implying completed. This has nothing to do with is being in the present tense. Rather, the "pastness" of this comes from the adjective done.

The same applies to finished.

On the other hand, the present tense can also imply something that occurs continuously, in no particular time-frame. This is the case for "the work is done by machines". Here, done is no longer the same done as the other one. It is a verb, that is continuously happening.

Parts of speech labeling may be contested here, but the general meaning of done as an adjective, vs done as a continuous action through the auxiliary "is" remains the same.

If I say, the "shrimps are killed," we can interpret this in the context of killed being an adjective, and the shrimps have died in the past. Alternatively, killed can be a verb in the passive, implying that the killing is a continuous action rather than one event. Both are very valid scopes with which to view the usage.

I guess it's up to context to clear the carnage!