Restating my assumptions:
- You need to send your spirit back in time to possess your earlier self to avoid execution.
- You have two sidereal weeks to prepare for this.
- You have no consequences in the present for your actions.
- You are in a very large city with access to real spellcasters.
- This is the climax of your game, barring that: "Our goal is to alter the past and survive three years until the moment the spell is cast, at which point I will be able to release the spell. "
- You'd quite prefer to win.
Let's start with solving for the trivial case:
Our goal is to alter the past and survive three years until the moment the spell is cast, at which point I will be able to release the spell.
We'll call this iteration 0.
You want to survive for three years. Taking that literally, the specification is for three sidereal years, not three years subjective time. You'll want to, this iteration, learn the precise tuning forks required for two demiplanes. Demiplane 1 has the flowing time attribute, and is preferably a nice garden or something. This demiplane has a very very high time ratio, let us say 1 second to 1 hour. (With that ratio, you'll want to spend 7.30484397 hours there for three sidereal years to pass.)
Demiplane 2 has the same flowing time attribute, but in reverse to whatever extreme ratio is allowed.
Hire a caster and manifester to do the following things (I am presuming your party can't do them.):
Manifest Hypercognition (three times) on top of the being's Knowledge (Planes) to find you your two tuning forks. 3600 gp. Call it 400 gp tip (for use of high knowledge skill plus whatever buffs they want to throw in.). 3000 gp. The third cast is to find a nest of Silthilar.
Make sure the caster who can craft wondrous items knows plane shift, craft contingent spell, last breath, and distilled joy. Bring extra casters if necessary.
Plane shift into your low-ratio demiplane (the one that gives you plenty of time). Either have the above spells enchanted into whatever spirit-stuff (I suggest calling them "spirit tattoos") will be sent-back, but fluff it however. Or have as many crafted contingent spells layered into your soul as you can afford. Given that magical effects carry backwards through time, the soul can be affected by magic. If necessary, plan elaborate scry & teleport robberies to pay the caster. You'll have plenty of time, no consequences, and whatever money you can acquire from a world that won't exist.
During this time, conduct interplanar negotiations with the nest of Silthilar. Offer for them to transport whatever research they want back in time for three years in exchange for free grafts on the other end.
Propose to your DM to just use Continuum time travel rules for this, as they're far more nuanced and coherent. Sigh when he/she refuses.
Spend the rest of the time engaged in whatever extra soul-enchantments are allowed by the remainder of your resources.
Go back in time 3 years. Let the first crafted contingent spell take effect and immediately port you into demiplane 1, with the high time ratio. Enjoy a nice book for 8 hours. Let the wibbly time spell elapse.
Swap over to demiplane 2. Spend as much time as necessary (reincarnating via last breath) to figure out how you did the time travel alteration in the first place. Cheese out knowledge checks as required. Then improve the time travel alteration to be an inherent property of your dragonmark. Get whatever other contingent spells/wonderous-soul-item thingies enchanted into your soul. Go back in time 3 years.
Win. you'll have detailed knowledge and research for the next three years of game time. You're in a time-loops scenario. Against any "leveller" (person who's boringly travelling level in time), you will win. When you encounter an unexpected threat, abort universe and iterate again.
Preserved as generic time travel advise:
The lack of items created yet poses a problem, as that criterion applies to your bodies as well. Still, given that something is moving through time (or, you're getting reassembled through information transmitted backwards.)
First, investigate how the Timeless trait interacts with your specific time travel rules. You might just be able to stick your stuff on an appropriate demiplane, remember its specific tuning-fork frequency, and retrieve on the other end.
Second, if stuff, but only your body stuff is moving, then you can get custom enchanted magic items which are slotless, embedded into your flesh. Which custom magic items is left as a negotiation with your DM, but there are an awful lot of first level spells that are absolutely awesome to be enchanted into magical items.
If only contiguous self-organic stuff is moving, look into grafts. While grueseome, get as many of these as your DM will allow. Some of the more risky routes can certainly be attempted within two weeks and can trivially consume all available resources. For a buckle-swasher, absolutely go attempt to contact the Silthilar, promise them to take all of their research back in time in exchange for free grafts, and take as many as they'll perform. If they require money, 100k won't even approach being enough. Still, the money is enough to find them. Otherwise, buy as many fiendish grafts as you can, and just make sure your cleric has lesser restoration prepared.
If only information is being transmitted back, perhaps you may want to start exploring grafting on Continuum to your game. In this instance, invest in crafted contingent spells up to the limit that your DM allows such that you can fake being prepared for your first dungeon crawl. Spend the rest on legend lore and other historical magics so you know what recently-dug grave to knock over to get a reasonable facsimile of your stuff back.
If only information is being transmitted back and your DM isn't being nice about finding a lair via legend lore and contingent spells aren't allowed... Go back further and spend an arbitrary amount of time with liquid pain + restoration powering your caster's recreation of magical items via torture. It'll take a while, but by definition you have as much time as you need. Consider instead ambrosia or any of the other techniques in this answer. Since you're going forearmed with knowledge, you can skip much of the initial bootstrapping, choose somewhere where you wont be bothered for a significant duration of time, up to and including specially found demi-planes for this purpose. Given that you have significant awareness of the past, finding a community that can support your intentions should be trivial.
If your DM isn't okay with self-bootstrapping via strategic information transfer leading to effective immortality, ask him/her what you're allowed to do, and do that. Given timetravel, there exist two possible time machines: the initial prototype and the best possible version. Given information transferring timetravel, there is nothing preventing you from heading back in time, porting to the appropriate location, ensuring you have enough resources for eternal life, spending as many years as it takes on perfecting the time travel ritual, and then heading back again to do things correctly.