Lydia can spend this much XP to spend:
Attributes:
At chargen Lydia had the following:
4 dots (16 XP to 5 dots): Intelligence, Wits; 2 attributes = 2* 16 XP = 32 XP
3 dots (12 + 16 = 28 XP to 5 dots): Dexterity, Charisma, Manipulation, Appearance; 4 attributes = 4* 28 = 112 XP
2 dots (8 + 12 + 18 = 36 XP to 5 dots): Strength, Stamina, Perception; 3 attributes = 3*36 = 108 XP
Total 252 XP
Abilities:
At chargen Lydia had the following:
3 dots (6+8 = 14 XP): Brawl, Empathy, Academics; 3 abilities = 14 * 3 = 42 XP
2 dots (4+6+8 = 18 XP): Athletics, Animal Ken, Etiquette, Investigation; 4 abilities = 4*18 XP = 72 XP
1 dot (2+4+6+8 = 20 XP) : Alertness, Awareness, Expression, Intimidation, Subterfuge, Stealth, Finance, Occult, Politics, Science, Technology; 11 abilities = 11 * 20 XP = 220 XP
0 dots (3+2+4+6+8 = 23 XP): Leadership, Streetwise, Crafts, Drive, Firearms, Larceny, Melee, Performance, Survival, Computer, Law, Medicine; 12 abilities = 12 * 23 XP = 276 XP
Total 610 XP
Willpower from 6 to 10 = 5+6+7+8+9=
35 XP
Charms (she's buying them at 3 XP * dot rating cost, and I'll substract what she had at chargen):
6 excellencies = 3*6 = 18 XP - her excellencies apply to her attributes, rather than her abilities.
10 1 dot charms = 10*3 = 30 XP - 1 charm at chargen = 27 XP
11 2 dot charms = 11 * (3*2) = 66 XP - 3 charms at chargen = 48 XP
8 3 dot charms = 8 * (3*3) = 72 XP - 1 charm at chargen = 63 XP
6 4 dot charms = 6 * (4*3) = 72 XP
4 5 dot charms = 4 * (5*3) = 60 XP
Total charm XP to spend 288
Prayer eating (I am honestly unsure at what rate Lydia would buy that; ExvsWod says only Infernals are attuned to prayers, but she's a demigod heir of a god, and that seems so very in theme for her):
30 or 45 XP. To be on the safe side, let's say
45 XP
Total total: 1230 XP
There's also stuff like familiars (which is clearly integral to Lydia's character design) and paths, but I'll leave them out. I'm also leaving out Ancient Sorcery, as I am unsure if Lydia, as a terrestrial-tier exigent of a being who wasn't alive during First Age, has access to it. For comparison Molly had 2378 XP to spend.
If we directly scale
essence progression by 1230/2378, Lydia's Essence progress chart would be
E3: 48 XP
E4: 112 XP
E5: 239 XP
E6/Elder: 493 XP
Currently on average Lydia got ~8 XP per arc (
8 after arc 3,
6 after arc 4,
8 after arc 5, and
9 after arc 6) to the total of 31 XP so far. If the trend continues, she'll hit E3 after two more arcs (i.e. around christmas time, which seems kinda appropriate in a way, I guess? Winter solstice would probably have some connection to death).
Strategy-wise, it seems to be that the best bet for Lydia would be to leave abilities as the last thing to buy. Invest in Willpower, buy the remaining excellencies (it's feasible for her), and raise all attributes to 5, starting with those she has excellencies for, intermixing charms in between. She'll be able to coast on rolling pure attributes pretty well, and using her near-celestial tier excellency for bursts of power. She'll be able to hit Elder essence (assuming the provided scaling) without putting a single point into abilities, which could be an interesting type of build, I guess. Supreme talent and ability, with effortless excellence, but not much dedication to specific areas. A "technical debt" kind of build, where you grow as powerful as possible, and then, with your very existence secured, go back and do formal education.