Hi, Thorondor! A couple of points to address here:
(1) Quenya names aren't generally formed from sentences in the way Joshua is, so your loose genitival construction "God-salvation" is on the right track.
(2) Note that sámë is a fan-made word (so Tolkien may have had a different idea altogether for "salvation"), and note also its long a would be preserved in construction: Erusámë (or Eruþámë, minding its derivation from root ATHA). Quenya names will often also incorporate a gender-indicative or genealogical suffix — say, Eruþámo or Eruþámion — but they'd also never incorporate the holy name of Eru, so I think we're OK to deal flexibly with an element-for-element translation of a real-world name..
(3) Unlike Sindarin (whose phonological history is daunting at best) Quenya has a conservative sound system that allows compounds to form fairly seamlessly most of the time. The "best" or "most proper" way to combine is to become more familiar with the language and its history at large. We're also a fairly robust and responsive community, so feel free to run names by us for accuracy or other ideas!