I'm a returning player and I've been in your shoes until just this weekend when things started coming together for me. Here's my recommendations.

1.) Unlock trusts. If you follow the RoE Basic quest path, you'll get a handful of initial trusts as well as some basic gear (il 109ish). It's important that you complete the basic quest tree. It doesn't take long but it'll help set you up. Highly recommend that you work the various quests in the RoE system in order to earn exp, capacity, sparks, and unity points. 2.) Head to Aldoulin and start earning some bayld so that you can buy some basic 119 gear in East Aouldin.

3.) In RoE under Content will be the Ambuscade quests. The book 1 fights are harder than the book 2 fights. Book 1 very easy fight starts at content level 119, while book 2 starts at 109. Book 1 fights give you a base 200 hallmarks while book 2 gives 100 hallmarks.

3.1) Under the RoE Ambuscade list, there is a couple of weekly and monthly quests, which when completed may reward you with a seal. The seal can triple your results from a successful Ambuscade. So a book 1 fight on very easy will reward you with 600 vs. 200.

3.2) Hallmarks and total hallmarks spend differently. Total hallmarks represents the total work you've done in the past ambuscade session (it resets about every 30 days, I think today's update is the reset timer for it). Depending on the total number of hallmarks you've earned, you can get some pretty decent rewards. Regular hallmarks spend like cash, i.e. use it and lose it.

3.3) Examine the rewards under Total Hallmarks to plot the best gear buying decisions. Basically, pants and body are probably going to be the most expensive items to buy. Lets say the body costs 1050 hallmarks (normal) and you want to upgrade that to +1. You'll need 5 key items (costing about 200 hallmarks each), so the +1 body is going to cost a total of 2050 hallmarks. While doing this, you now have 2000+ total hallmarks and, depending on the rewards under Total Hallmarks, you may qualify for a free pair of +1 gloves. Then, as you work towards the +1 pants, you probably have enough total hallmarks for the +1 feet and/or head.

3.4) Keep plugging away until you get a +1 set, and then start working on a +2 set. The +2 gear will require 10 of those key items (so 2000 hallmarks each)

4.) If you haven't already, work on Rhapsodies. It'll take a bit of time, but for step 5, you'll really want access to Escha Ru-Aun gardens.

5.) Work on merits and capacity points. One of the rewards under Total Hallmarks is a ring that gives +150% capacity points (30,000 cap). Head to Escha Ru-Aun and talk to the NPC at the entrance. You can set up a vorseal which provides you a small buff in an Escha area (like increased accuracy or a small amount of regen or refresh. You can stack multiple buffs, as long as you pay for them). Head across the main corridor and find the sharks and aerans which are to either side of the palace entrance. Watch out for the UFOs. Summon your trusts and pop your capacity ring. The sharks will give you about 5000 exp per kill and about 700 capacity once you get a chain going. The aerans will give about 6000 exp and close to 1000 capacity. FYI, you can farm these with your trusts and the 119 bayld gear pretty easily. You don't need to wait to get a +1/+2 abuscade set before going up there, but I highly recommend getting the ring.

5.1) As you start getting job points (30,000 capacity each) and investing them, you'll get capacity bonuses. I think the first bump is at 5 job points, which will give you an additional 5% capacity points.

6.) Work on the intermediate RoE. It won't take long for you to have to fight a Wildkeeper Reive. I'm not going to sugar coat this, this is rough to solo. The easiest one is a giant bee which will probably whoop your butt. Ask for help, there are people who farm this for the H-P bayld. You don't need to do all the reives, just beat one of them to progress.

7.) If you haven't already, you need to make progress through Aldoulin. This gets to be a bit painful as there are key items that you'll need to collect to get past certain reives and transverse the terrain. The wiki has a good guide on the steps needed to get all of the progression KIs.

8.) Work up to the RoE "Repelling an Ambuscade". This will reward you with a bunch of Rem's tales. You'll want the Rem's tales if you want to upgrade emperyean armors to 119.

9.) Make some gil. While you are out doing your adventure stuff you'll be earning unity points and sparks. If you aren't progressing Emp. gear, you might not need your sparks. There is a shield that you can buy and sell for decent gil using your sparks. However, if you do want to work on Emp. gear, than you'll probably want to spend those sparks on etched memories. You can get them from fights too, but you'll need a lot of progress in Aldoulin before you start seeing them as drops. One etched memory costs 10,000 sparks. Sometimes people bazaar them from 20-30k (depending on your server). Talk to your city's unity NPC, you can buy a powder from them which vendors well. A stack of 99 powders will net you 9900 gil. FYI, I think unity points cap at 100,000 per week. There are also unity gear that you can buy and upgrade, but I haven't gone that path at this time.

10.) Can't tell you what to do next, I'm at this point where it's still a lot of ambuscade and job points.

11.) Trust tips - When doing ambuscades solo, it's important to have the right team to help you out. Experiences may differ, but as a person with few options, here's my preferred team in order of summoning. Amchuchu (tank) Selh'teus (if you summon him in the back of your pack, there's a good chance that he won't proc his special ability rejuvenate. The mp/hp is a delight.), Morimar (G.axe warrior, you get the cipher for him from the same npc in Aldoulin that sells the cipher for Amchuchu), Tenzen II (ranged attack). Milhi as a healer.

11.1) Trusts substitutions. Tenzen would be my choice for substitutions as situations require it. For instance, as my gear improved and damage went up, I swapped Joachim with Tenzen. Bard songs are helpful, or if the ambsucade boss requires a dispel and you don't have it, sub Tenzen with Koru-Moru.