You can actually start your legacy item at 5th level. If you read the first sentence of the first full paragraph on pg 9, "A legacy item's wielder who completes a given ritual immediately gains a bonus legacy feat: Least Legacy, Lesser Legacy, or Greater Legacy." You'll be spending gold, XP and time instead of a feat slot to upgrade your weapon. Those feats are there just to prevent someone else using the additional abilities it provides.
Since you seem to want to found a Legacy Item, you'll need to pick one of the charts listed on pgs 185-186; my group has always been partial to 4-2 and 4-3. You'll also need to pick an Omen for your bow. While knowing your intended build is helpful, not knowing your particular play style nor what type of campaign you're going to be playing in makes generating a legacy item difficult. Personally, I'd would work with your GM to make this item... which will provide an avenue to gaining access to more customized abilities unlike the generic ones listed in the book. With that said, here is my attempt (will provide multiple abilities for a level, you pick the one that seems best).
Least
5 - Creature Compass (For monster themed games) / Lightbringer (If you don't have dark vision) / Magical Cipher (You gave up spell slots/known, the spells given are used often enough to warrant a consideration.) / Call to Hand (For a Move action, always have your bow ready to use.)
6 -
7 -
8 - Lesser Empower Spell / Scorching Ray (Caster level 10; you get AT WILL two 4d6 fire damage rays with a range of 50ft.)
9 -
10 - Weapon Enhancement (If your GM doesn't allow you just pay for an upgrade added. Should become +1 or +2.) / Web
Lesser
11 -
12 - Weapon Enhancement (Same as above. Should become +3.) / Invisibility / See Invisibility (Not worth your reduced spell known, but great if your GM likes to use invisible opponents)
13 - Glitterdust (If you skip See Invisibility) / Darkvision (If you don't have it already or the Lightbringer) / Cunning (Less sneak attacks coming your way.) / Major Intelligent Legacy (It can use it's own abilities, effectively granting another action each turn. It's alignment matches your's and you get to pick it's goal... with GM discretion. There will still be a risk of competing EGOs, but should be no where as dire other kinds. Definitely talk to GM before taking.) / Burning Hands (Caster Level 10th; you get AT WILL a 15ft cone of 5d4 fire damage.)
14 -
15 -
16 - Evasion / Poison Immunity (Stack even more effects/damage on your attacks.) / Weapon Enhancement (Same as previous. Should become +5)
Greater
17 - Wizardry II (To make up for your reduces spell slots.) / Lesser Quicken Spell / Weapon Enhancement (Same as previous. Should gain +2 worth of names abilities.)
18 -
19 -
20 - Wizardry IV (Same as before.) / Weapon Enhancement (Same as previous. Should gain +3 worth of names abilities, putting you at a +10 weapon.) / Time Stop (End game spell you won't have access to due to build. Caster Level 20th.)
From my experience the weapon followed closely by armor are the worst items to make into a Legacy Item if you are limited to just the abilities presented in the book. Both need to be upgraded/replaced every so often and some GMs are not inclined to allow you to simply pay for an enhancement upgrade on Legacy Items. That means spending a precious ability slots on said enhancement upgrades to keep up with the rest of the party (and monster attack bonus/AC).
There are also abilities listed on the examples Legacy Items that are not on the founding lists, like Call to Hand (pg 145).
Some examples of custom abilities I would request your GM to consider.
* Light Armored Mage: This ability allows you to wear light armor without arcane spell failure. While it does not negate the need for Spellsword it does allow you to wear a Chain Shirt instead of Leather. Would consider this as an A or B level ability.
* Expanded Imbue Arrow: This ability would allow you to use Touch spells and/or Ray spells with the class feature Imbue Arrow. Would consider this a D level ability, but you might be able to argue a C too.
* Imbued Scorching Ray: This ability requires you to take the Scorching Ray ability listed above. As a Standard action, you can shoot up to two arrows, each deals the same damage as a Scorching Ray from the required ability in addition to the damage the arrow would normally do. This is used in place of the normal Imbued Arrow class feature. Would consider this a D level ability.
* Imbued Wand: If you can convince your GM to allow a Wand Chamber (Dungeonscape pg 34), this ability consumes 2 charges of the stored wand in place of a spell slot when using Imbued Arrow. Normal restrictions of spell usable still apply. Would consider this a B or C level ability.
* Extend Arcane Boost: This ability increase the duration of the class feature Arcane Boost by half the spell level (rounded down, minimum 1) of the expended spell slot. EI. a 1st level slot would grant to 2 rounds and a 5th level slot would grant 3 rounds. Would consider this an E level ability.
* Cluster Shot: When you use a full-attack action to make multiple ranged weapon attacks against the same opponent, total the damage from all hits before applying that opponent’s damage reduction. Would consider this a C level ability.
Last but not least, I personally would consider the Eldritch Knight over the Legacy Champion. With this change, it will increase you BAB from 17 to 19 (which makes your effective caster level higher); increases your caster level from 15 to 16 which grants you access to 8th level spells and grants your a bonus fighter feat. Even with your misconception of how the Least/Lesser/Greater Legacy feats work, you could have still paid for the Least and Lesser rituals before obtaining the first level of Legacy Champion. The same applies to the Greater Legacy ritual is available three levels before Reduced Ritual Cost (Greater). The Bond of Lore is only useful if you find a preexisting Legacy Item instead of founding one like you seem to want to. The Extra Legacy ability use (least and lesser) are useless with the list of abilities I presented, everything is continues or at will. In the end the only perks you'll see out of the class is a d8 vs d6; 2 more skip points per level; +2 will instead of +2 Fort; one Bonus Feat (I recommend Quicken Legacy) and Replace Legacy Ability (Least & Lesser) assuming you can find/make one that is a better fit for the game.
