William Seamus is too M-heavy.
I think
Seamus is a nice Irish name. And almost any name is good as a mn. A mn does not have to be modern or easy to pronounce.
"You sought a flower and found a fruit. You sought a spring and found a sea. You sought a woman and found a soul. You are disappointed."
"It does not become me to make myself smaller than I am." (
Edith Södergran 1891-1923)