First of all, Thingol did fight Morgoth, while the Noldor were busy murdering and stealing and betraying each other. He allied with the Laiquendi, Falathrim, and Dwarves to battle Morgoth's forces. You could almost call it... a Union! And his army won a partial victory without special Tree-light powers. But by the time the Noldor came, he'd retreated because his forces were decimated (and he had no idea that "help" was coming). That, coupled with the remaining Laiquendi refusing to fight, means that Doriath likely did not have much of a standing army.
The Noldor arrived under mysterious circumstances, were secretive yet clearly showed internal factions, highly militarized, had explicit intentions to rule Beleriand without regard for the Sindar who already lived there, and insulted Thingol in a specifically racial manner. Not off to a great start! And then he learned about the Kinslaying and Doom of Mandos that they hid for decades!!
The Doom that specifically states: "On the House of Fëanor the wrath of the Valar lieth from the West unto the uttermost East, and upon all that will follow them it shall be laid also... To evil end shall all things turn that they begin well; and by treason of kin unto kin, and the fear of treason, shall this come to pass. The Dispossessed shall they be forever."
So, from the beginning of the First Age, Thingol was surrounded by Noldorin warlords, half of whom posed an implicit threat to him and his people, and he lacked the material power to keep them in check. He lacked that power because he fought Morgoth and suffered heavy losses. Even so, he sent his Marchwardens to aid the Haladin and welcomed refugees from the Bragollach. I take the quote: "The most part of the Grey-elves fled south and forsook the northern war; many were received into Doriath, and the kingdom and strength of Thingol grew greater in that time" to mean that at that point, Doriath did have more manpower, but I'm not sure how many were willing to fight.
Thingol then set the Silmaril quest. That was stupid. He should not have done that. But let's look at what happened: Luthien was kidnapped and assaulted, Finrod was usurped, and the implicit threat became explicit. This is the background for his (and most of Doriath's) refusal to join the Nirnaeth. I'll let the text speak for itself.
"From Doriath came little help. For Maedhros and his brothers, being constrained by their oath, had before sent to Thingol and reminded him with haughty words of their claim, summoning him to yield the Silmaril, or become their enemy. Melian counselled him to surrender it; but the words of the sons of Fëanor were proud and threatening, and Thingol was filled with anger, thinking of the anguish of Lúthien and the blood of Beren whereby the jewel had been won, despite the malice of Celegorm and Curufin. And every day that he looked upon the Silmaril the more he desired to keep it forever; for such was its power. Therefore he sent back the messengers with scornful words. Maedhros made no answer, for he had now begun to devise the league and union of the Elves; but Celegorm and Curufin vowed openly to slay Thingol and destroy his people, if they came victorious from war, and the jewel were not surrendered of free will. Then Thingol fortified the marches of his realm, and went not to war, nor any out of Doriath save Mablung and Beleg, who were unwilling to have no part in these great deeds. To them Thingol gave leave to go, so long as they served not the sons of Fëanor; and they joined themselves to the host of Fingon." [[Yes, he should have listened to Melian, but I'm not going to equate his stubbornness and foolishness with the Feanorians' open threats of another kinslaying.]]
Moreover, Thingol knew that only the Valar could defeat Morgoth, and he may have heard from Cirdan that Turgon was trying to send mariners back to Valinor for aid. And to recap: he knew about the Doom of Mandos and the risk of being entrapped in it. He was blind to the fact that he was already entrapped by the Oath but, well, I never claimed he was perfect.
TDLR; there was absolutely no reason for Thingol to join the Union of Maedhros and, even if he had, they still wouldn't have won. He most definitely made mistakes and did some horrible things- "abandoning Beleriand" was not one of them.
