Then the question(s) becomes 'do they get Eärendil circa War of Wrath, with Vingilotë, the Silmaril, the accompanying flights of Eagles? Huan and Melian, divine beings who were nonetheless part and parcel of the various messes that happened in the actual First Age?'
If so, then it basically boils down to 'can Thanos with one or two Gems beat this group?' because he's it. Even with the size difference, Huan would probably beat up MCU Fenris (by 'probably beat up', I mean 'probably kick his ass', not 'probably win', he's gonna win), which puts him somewhere above pretty much everyone on Thanos' team outside of Thanos, himself. Lúthien puts people like Morgoth to sleep, Galadrial personally blows up Sauron's summer home, Melian enshrouds entire countries with a mist that leaves her enemies completely lost and confused while people FROM that country are just fine, Eärendil and Manwë's Eagles kill all crap out of a horde of Dragons led by Ancalagon (hey, look, it's Thanos' big immobile ship, which is nonetheless really no worse than a dragon who wrecks three mountains in his death throes)…
…I'm taking a wild stab in the dark that, outside of Eärendil and the Eagles (mentioned) we're not getting any of the Host of Eldar that the Valar sent to fight in the War of Wrath? Because even without the Maiar, that's thousands of Valinor Noldorin Eldar on top of all of the legions who already exist in Beleriand through the First Age (the Leaguer, the forces of Gondolin and Menegroth, the Dwarves of Belegost and Nogrod, the Sindarin warriors of Doriath, the many Houses of the Edain, etc etc). Plus the Vanyar Eldar, who by all accounts make the Noldor nervous when it comes to beating stuff up. Probably best to leave them out, but it doesn't make THAT much of a difference.
Basically, even without the Eldar hosts from Valinor, there is one crapton of armed and armored superwarriors ready to fight Thanos' armies. Layer on top of that some extremely powerful individuals AND some weapons of mass destruction (Huan, Lúthian potentially, Eärendil/Vingilotë/the Eagles), and Thanos' ground-based forces that we see in Endgame are pretty outnumbered and screwed.
It's coming down to 'Can Thanos pull a moon down on them or something?' At which point, we're getting into 'war of powers' so it's kind of odd to allow Thanos to keep that kind of stuff while the Valar are disallowed. ^_^ But stuff like Thanos's moon feat is a tad beyond anyone except, I don't know, Morgoth at his height or the like, so if he gets the ability to basically carpet-bomb his enemies with impunity he unsurprisingly wins.