The romans were filtering germanic practices to dieties and concepts that were familiar to Rome.

Much like language, the pantheon of Indo-European cultures have a common ancestry and traceable path of decent. Almost all extant european languages descend from Proto-Indo- European (PIE). Overtime, some divine figures overlapped, took on aspects of other dieties, came in and out of favour, and lost or gained characteristics. However, despite their original bronze age/ late neolithic roots, there tended to be specific archetypes that were similar or even identical between the cultures the diversified from PIE. In short, once upon a time, romans, germans, greeks, celts, gauls, persians, hittites, sythians, and many others were all one group of people with the same language and gods.

We can't say with absolute certainty, but in all likelihood, Herucles was a reference to Thor. A son of a chieftan God, and a heroic "big brother" type character. Revered for feats of strength and battle against monsters. The stories about both characters differ greatly, but likely descend from the PIE god Thunor, a god associated with and named for thunder. Occassionally with metal working, In the Greek pantheon, the "role" of thunor may have split into herucles, zeus, and hephaestus. Zeus was a thunor descended character that filled the role of allfather. Because religions are a mess.

As far as Mars, that one is easier to equate. The role of a Warrior God was often filled in germanic cultures by Tyr, also called Tiw, and in PIE Tiwaz and were equitable to Roman Mars and Greek Ares. This is the god who gives Tuesday it's name sake. The role of a god of war differed and was adopted in an out of the idenity of other gods over time. In the Anglosaxon pantheon, Tir, Ingvi, and Freyr may have all been very similar or identical gods at different times. Later into the viking age, most germanic pantheons saw Wotan/Odin become both a Chieftan and War god, when in older times he was more of a god of death and wisdom while gods like Freyr and Tiw had previously filled those roles. The norse stories of the Vanir-Aesir war may be a cultural relic of these social and religious changes over time.

To your question about Isis, this one is more hotly debated. There are a large number of goddesses that could be the candidate. Freya, Eostre, Nerthus, and similar, local goddesses were worshipped among germanic cultures as divine mothers, divine feminimity, magic, female leadership, and sexuality and love. In my inexpert opinion, worship of Eostre is the most likely candidate for the germanic Isis. Her cult and versions of it was popular in mainland germany and anglosaxon england. Relics of eostre worship are present in Easter, her association with rabbits, eggs, and the vernal equinox.

It's possible that there is an older connection to Isis. Hecate was a very similar goddess in the greek pantheon possibly directly inspired by Isis. It's possible that PIE pantheons may be related to the egyptian deeper in prehistory, and it's possible that egyptian beliefs may have inspired the ancestors of PIEs. However, the point is, pre migration era germans almost certainly did not observe a contemporary egyptian isis cult and decide to mimic it and no other part of egyptian religion. They had similar goddesses from the PIE family of dieties to begin with, worship of which was better documented in later times.

Isis was also very popular in the meditereanean, so germanics may have gotten her or a similar goddess second or third hand from a greek or roman influenced culture, but this is less likely than tacitus spitballing a stand in IMO.

Talking about any anthropological comparisons is difficult, because at a certain point, all humans have a common ancestor, and no culture existed in a vaccuum.

TLDR:/recap Most extant european, near eastern, middleeastern, and indian cultures once shared the same religion and language before diversifying over thousands of years. This is called Proto-Indo-European. It's why pagan pantheons in europe have similar gods, with similar traits, and some similar stories, along with interesting linguistic cognates that exist due to common decent and not borrowing. For a roman author, Herucles was the most familar stand in for Thor, Mars for Tiw/Tyr/Tiwaz/Ingvi/possibly proto Freyr.

The idenity of the Isis of the Suebi attested in Tacitus is not certain, but was almost certainly not directly related to Isis. Eostre, Freyja, Nerthus, and local variants are potential candidates. Similarities might be projection or misinterpretation by Tacitus, a deeper connection than history can account for, far older than the scope of specific cultures, or a local goddess indigneous to a germanic culture with no convenient stand in of the roman religion of tacitus's time.

Gods and Goddesses changed over time, took on or lost different characteristics, fell out of favour or resurged, sometimes generationally, sometimes between centuries, sometimes between one mountain range to another. It's both difficult to get specific, and it's very difficult to accurately generalize ancient beliefs when they're often documented by people with agendas, cultural misunderstandings, and centuries of separation, and the interpretations of historians looking at relics through the lense of modernity and their own biases and limitations.

I'm not 100% certain on the German archaeology but, by the peak of the Roman Empire, Isis was an established part of the Roman pantheon and so you will see her cult appearing in places as diverse as the Sudan and Northern England. There certainly will have been adherents of Isis in Germania.

What else is interesting is that the correspondences syncretism (ie: "the German Mercury") that happened with other deities did not seem to happen with her but instead her cult swallowed up local goddesses that were compatible with her functions as mother and initiatrix. That's possibly because there were "mysteries" attached to Isis rather than simply local cult activities. It's worth remembering that all this belonged to Greece much more than it did the Egypt of the Pharaohs.

Burkert's book Ancient Mystery Cults has a chapter on the Isis mysteries and is still the best academic introduction to the subject.