What would a correct translation for "Ashbringer"? 'Lith' for 'ash', but then the 'bringer' gets tricky.
Taking both Aryante ("Day-bringer") and Yavanna as precedents, could be "Lithante" for the femenine and "Lithanto" for the masculine? Would "Lithanna"/"Lithanno" even work?
Q. tultar ⪤ S. tollor (or tolthor) are options for "bringer".
Aryante and Yavanna are Quenya. So if you wanted Sindarin for Ashbringer. Would more than likely yield Lithollor or Lithosor due to dissimilation of spirants; [θSθ] > [sSθ|θSs].
But wait for others to chime in.
I don't think Lithosor is likely, since intervocalic s does not really occur in Sindarin. Also I do not think lith + tolla- would reduce to ...th... - compare Hithdilias < hith + tilias.
If it's intended as a late formation, I don't think dissimilation is necessary, so it could simply be Lithdollor. If an old formation, perhaps Lisdollor can be expected, but this is more uncertain.
An alternative option is to follow the example of bes[s]ain "breadgiver", which is cognate to Q. massánie (note that ante in Aryante is also technically "giver"). Here we essentially see -ain < *ānje. With this, we get Lithain "ash-bringer". It can also be made gender specific, masculine Lithónon / Lithonon, feminine Lithóneth / Lithoneth.