Avoiding "was able to be" in the passive voice

Considering https://dictionary.cambridge.org/grammar/british-grammar/could (point "Could + smell, taste, think, believe, etc."), I assume that one is allowed to use "could" to refer to single achievements in the past when the sentence is in the passive voice.

For instance:

The probability of obtaining suboptimal solutions could be lowered by simply making the optimiser search more but similar waveforms

is better than:

The probability of obtaining suboptimal solutions was able to be lowered by simply making the optimiser search more but similar waveforms

Two points:

1. obviously, only the context helps the reader to know that "could be" does not mean "is likely to be/may be". In the sentences before this one, I used exclusively simple past, so the tense should be clear in my humble opinion.
2. the latter sentence also causes a problem since "able" is used for a thing, which is usually reserved for living beings (at least in standard English as far as I am aware)

For me, "be" is also a stative verb like "smell, taste, think, believe" mentioned in the Cambridge article. I know that I can rewrite the sentence and use phrases like "it was possible" or "one managed to", but I would like to avoid these here.

So am I right in thinking that one can use "could be" in the passive voice to mean "was able to be", which sounds awkward?

PS: I am already aware of this article here: "was able to" vs "could", but it didn't help me in this regard. Help would be much appreciated!

You are referencing the wrong sense of could (single achievements in the past) in your dictionary.

Your sentence uses could in its sense of past (or reported) ability—in the same way that we use can for present ability.

The voice—active or passive—has no bearing here.

Using your dictionary, here is the relevant entry for could:

past simple of "can", used to talk about what someone or something was able or allowed to do
_{Source: Cambridge Dictionary—could (CAN)}

Let's look at some simplified sentences for illustration . . .

Present ability (active): Researchers can lower that probability by making the optimiser search more waveforms.

Present ability (passive): That probability can be lowered [by researchers] by making the optimiser search more waveforms.

Researchers currently have that ability. Are you with me so far? Next . . .

Past ability (active): [In the past] researchers could lower that probability by making the optimiser search more waveforms.

Past ability (passive): [In the past] that probability could be lowered [by researchers] by making the optimiser search more waveforms.

Researchers had that ability in the past. Following?

We also "backshift" can to could when reporting, even if the ability still stands; researchers still have this ability even though we're using the past tense:

Reported ability (active): [They observed that] researchers could lower that probability by making the optimiser search more waveforms.

Reported ability (passive): [They observed that] that probability could be lowered [by researchers] by making the optimiser search more waveforms.

If you apply context to your original sentence, it likely matches the example immediately above—reported ability in the passive voice.