It's quite unconvincing, to say the least : most sieges in Vth century Gaul were generally open field battles ending up with cities opening their walls to the victor.. That Syagrius opted for not remaining in a city (especially as his loyalties might be judged not that firm) when Gundobald is said, later in the text, to be uncomfortable being so due to logistical matters, doesn't strikes me as a strong evidence about his political power.
It's really a sur-interpretation of not a sentence, but part of it : while Syagrius did "not put off" a fight against Franks as he was not afraid, according Gregory, he quickly changed his mind at the defeat of his army : since there is no mention of reinforcements from other regions or men (as Gregory points Clovis had or expected), the only information we have was that Syagrius that ruled from Soissons (a former imperial arsenal) had this army and that's it.
That it was evenly matched (or not) can't be found or implied in the original text.
Again, this is a sur-interpretation IMO : Clovis as the leader of the Roman army of Belgica was certainly gathering it (including other foedi, on which Merovingian seems to have some ascendent). We do know, by other documents, that Clovis did have the military command of this region, something we don't have for Syagrius and is not attestable from the text.
With this kind of wild guessing, we could as well imply that Goths had a smaller army than Franks because they were afraid of Franks as Grégory puts it.
It's possible, although not mentioned in the text which would be weird coming from Grégory which tries pretty much to put Clovis in the best light possible without outright lying : Chararic is literally said to have waited for the victor, and that's it.
But even an alliance doesn't imply at the latest a form of regional dominance from Syagrius, even less in Belgica.
It's not what appear from the text at all : Gregory stress the relative easiness of Syagrius being sent back to Clovis in chains, when Franks couldn't have been yet a threat to Goths.
That's a good question : there's no clear answer. My take on this was that the judicial power of Clovis over Franks was clearly established and acknowledged, but as a Barbarian regulus not yet acknowledged by Gallo-Roman nobility wholly before his conversion, he might not had the legal right to execute or punish Syagrius.
Nothing attested in the text, of course; although the proclamation of Clovis as honorary consul and "glorious king" in Orléans might echo this change of status.
Gregory generally follows a chronological narrative there, and he gives in the text some markers for this. "Meanwhile a lot of churches were plundered of his armies" and a bishop ask that a rich object be given back to him, Clovis asking him to follow him to Soissons. Either Gregory completely changes his style at this point, either we assume that the plundering of Soissons lasted for years, either it's in the right chronological order and the execution of Syagrius happened roughly at the same time than Clovis' conquest of the region.
As for why Gregory doesn't hesitate to put Clovis in a bad light : his work was essentially a didactic history for the grandsons of Clovis, hoping to point why a Christian conduct is preferable for a ruler, with a contrast with Clovis' attitude before and even after his conversion : trying to lying would have weakened his objective, if not conscience.
If we take in account what follows, that in the tenth year of his reign Clovis took Tongres, it most probably took place before 491. At this point, any reason why it should have happened later than how Gregory tells us how it happened seems a bit moot.
The use of the plural may simply hints as several local rulers. I do not disagree that Syagrius might have enjoyed some sort of ascendency over other Gallo-Romance dux and leaders, but there is simply nothing hinting at that specifically.
In 476? It could as well be Sidonius Appolinaris which beneficied from strong ties with senatorial elites in Italy.Again, I really don't think it's the case myself, but with this kind of meager sources only, any claim can be made.
It's not because they controlled the region that local Gallo-Roman nobility ceased to be a thing : if anything, the Gondovald's revolt one century later does points at its dynamism and political activity.
Mostly because it's the only name we got in Northern Gaul, which is not the same than an actual mention or knowledge there was no one else : at this point we could argue that southern Britain was unified because we only have an handful of names succeeding each other.
More seriously, that Gregory specifically mentions Syagrius and no one else in Northern Gaul certainly points that he might have beneficied from a special ascendent in the region, probably out of his familial ties and power in the region. And that Chilperic does supports eventually Odoacer's while Syagrius would not is a good explanation as for the break of the alliance tied with Aegidius in the 460's.
It's quite possible than defeating Syagrius really helped Clovis to enforce his rule over Saxons and Alans of the north-western shores : but it's not clear how quickly or nt Clovis did so. One of the few names we got outside the Frankish feodus and its extension in Northern Gaul is in Trier with the count Arbogast. He was not under Chilpéric or Clovis' dominance originally as far as it can be told, but it was eventually part of the Frankish kingdom in the VIth without any indication how and when. Similarily, the Frankish expansion west of Seine and north of Loire isn't really well attested: Gregory simply doesn't mention anything besides the takeover of Soissons itself.
For all we know, after the Battle of Déols, Armorican region was let to itself between remaining Gallo-Romance leadership, remaining Bretons, Saxons and Alans : the attested Frankish presence and projection beyond their foedus is a valid explanation to a quick but gradual expansion in Northern Gaul, more than the conquest of an non-attested Gallo-Roman polity from Brittany to Rhine.
There's at least one logical step that got passed by there : somebody minted coins -> Syagrius maintained administration where they were found. Maybe there's more to it, but I don't have the book right now : I'll try finding it.
I'd want to mention that Clovis doesn't seem to have taken most of Northern Gaul after his victory against Syagrius : Soissons, obviously,
See, this is why I love late antiquity, it’s very generally a confused and source-light period where we are left to piece it together from the words of priests and token historians that are passed down to us.
In all likeliness, you are probably correct. I’d need to do some more research on the topic to continue the debate, at risk of derailing the discussion here. Anyways, back to your original response to my comment, what effect would receding Visigothic power in Gaul have due to a defeat at Déols? You’d have Roman and British armies operating in tandem to drive out the Visigoths. Could such a victory strengthen Julius Nepos’ position back in Italy if he claims responsibility for the victory?