"endless thanks" is correct and usable in written English. You can use it when expressing deep appreciation, such as, "We offer endless thanks for your kindness and support.".
Endless thanks.
Endless thanks to Winsome, Vickie, Gaby, Alicia and Jane, and everyone else who made recent years so happy.
Endless thanks to everyone that's helped along the way and immeasurable love to the incredible team that brought Showroom to fruition — Jeremy Anderson, Laura Ellner, Christopher Morben and David Gonzalez.
Endless thanks to my parents.
McMansion communities like Rancho Santa Fe have also helped recreate the golden state's car headaches and endless sprawl, thanks to planners and policymakers who have repeated the urban design sins of developed countries.
Nowhere is this truer than in Belgium, a country whose Dutch- and French-speaking populations tolerate each other (just), thanks to endless fudges and deals lubricated with taxpayers' money.
Cars jam the freeway, thanks to endless repairs to the crumbling flyovers.
Being a terminologist, I care about word choice. Ludwig simply helps me pick the best words for any translation. Five stars!
Maria Pia Montoro
Terminologist and Q/A Analyst @ Translation Centre for the Bodies of the European Union