@ Mr. Ferencz Indeed, I did run into that problem a few time when translating a few of my friend's names for fun.
For example if you look at the etymology of the name "Lindsay", I'm still unable to come up with a Sindarin equivalent that isn't a mouthful even for a person experienced with pronunciation (Names at 4 syllables w/o counting endings like "n(d)ír"/"Ion" tend to feel ridiculous, as you mentioned).
@ Mr. Calaerchon Yes, you could theoretically do both. As Tamas said names of extremely old meanings can turn out to be a little odd and really the older they are the more connotations, possible meanings, and general condensing may have accumulated/occurred. The Eldarin custom of translating their names makes sense from their perspective because, as Tamas said, their names were chosen for their meaning, and when going to Sindarin (Vs. some real world languages to Elvish) the names generally didn't increase in length. E.g. "Q. Fëanáro to S. Faenor", "Q. Carnistir to S. Caranthir (Alt. Cranthir)".
Assimilating the sounds of the name also works: adapting my earlier example "Stefan" to Sindarin.
By Etymology you may (Rinír may work too) get Ríon, by Assimilation you (may) get something like Thefán/Thefon.