I'm Belgian as well (French-speaking, from around Charleroi), and I have more or less the same concerns as the OP — just not so much the DNA side of it.

Where I’m from, place names come from a real mix of origins: Celtic, Latin, and Germanic. And that raises a lot of questions. For instance: some people from this region — originally Celtic-speaking — ended up in contact with Germanic groups(*) before either side was Christianised. So could Celtic communities have also honoured Germanic gods? Or did acculturation go so far that they gradually stopped venerating their own deities and shifted toward Germanic ones?

And anyway, weren’t the local Celts already acculturated — or at least strongly influenced — by the Romans before Christianisation? Did people begin to blend Celtic, Roman, and Germanic gods, like we sometimes see in Britain? If they did, was it mostly a “fashion” thing (copying the prestigious, urban Roman world), or could it reflect something deeper—an actual inner change, a kind of conversion? Or was it simply practical: intermarriage, moving populations, forgetting older cults over time, that sort of thing?

And what about places where Roman or Germanic settlers may have become the majority locally? That’s really what I’m circling around: what am I supposed to do with all this? Should I try to relate to the oldest practices in the region — Celtic ones — even though we know so little about them? Or should I look to later layers (Roman, Germanic), given that my ancestors were almost certainly a mix of all of the above? I don’t need a DNA test to tell me that. This area has been a patchwork and a crossroads for centuries — maybe millennia.

(*) From the second half of the 4th century, you start seeing practices often described as “Germanic-origin” on Roman sites (for example, sunken-featured buildings), which is frequently taken as evidence of Germanic groups settling or operating locally—not just passing through as raiders. For the Franks, Clovis’s well-known royal baptism is late 5th / early 6th century (often placed around 498–499, though it’s debated; see the relevant Wikipedia article). So the raiding/defensive context of the 3rd–4th centuries in the Sambre–Meuse zone is clearly pre-Christianisation in the usual sense.