Way back in Germania (first century CE), Tacitus notes that the Suebi venerate "Mercury", "Hercules", "Mars", and "Isis". These are generally interpreted by scholars as referring to early Germanic versions of deities we refer to today as Odin, Thor, and Tyr, whereas the identification of "Isis" is the matter of some debate.

The reason for this is not only because of how Roman 'interpretation' of deities often works (a specific process refer to by scholars as Interpretatio romana), but because some centuries later Germanic interpretation (Interpretatio germanica) interpreted Mercury as Odin, Mars as Tyr, and Jupiter as Thor based on similarly perceived similarities (the Germanic weekday names—including all but one English weekday name).

Several centuries later (around the 6th or 7th century), we find references in Elder Futhark to Thor and Odin on one of the Nordendorf fibulae (and possibly even Loki, in fact). Around this same time, Jonas of Bobbio writes that an earlier missionary had interrupted a beer sacrifice to Odin in Swabia, who he notes others refer to as Mercury.

Later, text found in a manuscript from the 9th century, known as the the Old Saxon Baptismal Vow, also mentions Odin, Thor, and a deity known as "Saxnot", an unclear deity.

An especially notable item from what is today southern Germany is a manuscript containing two very pagan charms composed in Old High German, which we today call the Merseburg Charms. These reference not only Odin but also Balder and four goddesses and a collective group of women, the idisi. Of the four goddesses mentioned by name, three are attested much later in Old Norse sources (Volla/Fulla, Sól/Sunna, Frija/Frigg), whereas the identity of the fourth is unclear (Sinthgunt).

In addition, we receive some insight into the religious pre-Christianization religious beliefs of the Lombards by way the Lombardic foundation myth, attested from the 7th century, which provides a narrative about Frigg and Odin that is quite similar to what we find in the prose introduction to the eddic poem Grímnismál, recorded in at latest the 13th century.

There's quite a lot more to say about this region but, in short, scholars have historically interpreted this material as closely paralleling the much more expansive material that comes down to us from the North Germanic record, mostly Old Norse material attested from the 13th century. Given how closely this material resembles the much later material, it's easy to see why, but it's also wise to expect some level of regional and chronological difference that, due to the state of the record, is difficult to detect.

For discussion on the ancient folklore of this region, you're best off becoming very familiar with the North Germanic record first. A great place to start with this is well-known folklorist John Lindow's 2020 "Old Norse Mythology" (Oxford University Press: https://global.oup.com/ushe/product/old-norse-mythology-9780197554487?cc=us&lang=en& ). From there I recommend checking out his similarly named handbook. Both are mercifully cheap and easy to come by.

If you're looking specifically for early Germanic reconstruction, you'll find the going rough without a serious background in historical linguistics, but you can still follow the breadcrumbs. Here's a handy article for that: https://en.wikipedia.org/wiki/Proto-Germanic_folklore

And for more discussion on this topic, you'll find r/AncientGermanic handy.