It is really sad to read about those natural catastrophes and I am very sad by all the deaths around this event.
I would like to comment about the scientific article you mentioned (focusing on the science part only).
If I am not wrong, it could be a coincidence that the model the authors introduced worked, couldn't it? It may be a very naive comment (I am by far not an expert in this field), but I thought there was no satisfactory way to model the occurence of earthquakes and tsunamis. There is no unique model that works in all cases and that allow to predict them long in advance. In other words, I assume we could find dozens of other models whose associated predictions fail.
It appears that they accurately predicted the type of collapse of the volcano and the magnitudes and timing of the tsunami waves that resulted from the cataclysm. Certainly they did not predict the exact timing of the event, although there was significant volcanic activity 2 months earlier, which should have triggered warnings:
https://www.volcanodiscovery.com/photos/krakatoa/eruption-oct-2018/explosion.html
In terms of predicting timing of volcanic eruptions (and resulting tsunamis) I agree that there is no way to satisfactorily model their occurrence. Broadly speaking, however, there seems to be a connection between volcanic activity and sunspot activity, and since we are entering a grand solar minimum, we should anticipate increased volcanic activity:
http://adsbit.harvard.edu//full/2003ESASP.535..393S/0000393.000.html
A possible cause of the association between volcanic activity and grand solar minima is proposed to be cosmic rays:
https://www.xyz.net.au/astrophysicists-link-volcanic-eruptions-grand-solar-minimum/
Thanks a lot for all the references. Yes, people should at least be prepared to react quickly if needed. There are countries (I think about Japan) that are probably much more prepared than others. by virtue of a political will to make the country prepared. This unfortunately does not seem to be the case in Indonesia :(