It is difficult to gauge the proper rarity.
The most powerful Rare weapon that requires attunement is likely the Sunblade. The Sunblade shares the +2 bonus but does not have a conditional 1d6 in addition. The damage output is as follows (since the condition for 1d6 is easy to meet, I reduced the damage for comparison by 0.5, a method I have used many times in the past).:
- Sunblade: 1d8 [4.5] + 2 = 6.5 (requires Attack action or Dual Wielder)
- Elven Thrower: 1d4 [2.5] + 2 + 1d6 conditional [3.5 - 0.5] = 7.5 (can be used with Two-Weapon Fighting)
Being able to use the weapon with two-weapon fighting is extremely strong and effectively increases your damage output by 3.5 - 0.5 (the other light weapon you are wielding, easy condition) bringing the total to 10.5.
One of the only times radiant damage is better than piercing damage is if you are fighting swarms, plants, or some undead (since it is a magical attack). Hard condition to meet (unless the setting warrants otherwise) means +0.5 for the Sunblade comparatively. We end up with 7 vs 10.5. Sunblade does however keep a hand free, an easy condition to use that hand (for a shield or spellcasting) for +1.5. The difference is therefore 2 points, which is somewhat decreased by the light from Sunblade.
Compare to Scimitar of Speed, a Very Rare item.
- Scimitar of Speed: 1d6 [3.5] + 2 = 5.5 (gets bonus action attack)
- Elven Thrower: 1d4 [2.5] + 2 + 1d6 conditional [3.5 - 0.5] = 7.5 (can be used with Two-Weapon Fighting)
Again the bonus action attacks increase the damage output: Scimitar of Speed has a free condition so +5.5 = 11; Elven Thrower has an easy condition so +3.5 - 0.5 = 10.5. Scimitar of Speed does however keep a hand free, an easy condition to use that hand (for a shield or spellcasting) for +1.5. The difference is 1.5, fairly comparable to the difference from the Sunblade.
Elven Thrower is right in the middle of Rare and Very Rare, so you should be fine at Rare, but it will be slightly above the power curve.