@Blue Wow great and thorough comment, it's very appreciated.
As of current, I believe my DM would be on the generous side. I.E. My beast keeps leveling as I do.
I think stopping the ranger levels around 5 - 7 is my best bet.
Level 5:
-Cordinated Attack - Reaction Attack for my beast
-Level 2 Spells
Level 6:
-Greater Favored Enemy
Level 7:
-Beast’s Defense - Advantage on ST's for my beast
Once I switch back to rogue, I'm not ssure which Archetype is the best to go for. What your opinion Blue?
And, I get my first ASI/feat at next level. What do you recommend?
If your DM gives you beast progression with character levels then you've got the right idea - get it a bit more buffs and then enjoy another class while the beast keeps growing.
Just looking at what you get at 3rd - the next subclass feature isn't until 9th, and that's 16th for you if you take Ranger up to 7th first.
As a side note, if you do take ranger up to 7th, might as well go 8th for an ASI (which your beast also gets) and you also get Fleet of Foot which helps you stay out of melee range.
I know you were originally looking at Assassin, but there's a lot of overlap with some of the signature assassin features, such as advantage on the first round. Being able to crit if you surprise depends on your party and tactics since that isn't initiative dependent but usually stealth and circumstance.
Thief doesn't seem to really bring you much for your theme, though it's a decent subclass.
Arcane Trickster could be nice if it gave you buffs for your beast, but with the exception of one spell in your initial, all the rest need to be enchantment or illusion. Also many of the spells are saves so you need a decent Int.
SCAG gives us the Mastermind, which you'd have to figure out if that fits this character's style. It gives the ability to do the help action as a bonus action at 30' - for the times you are that close it can boost your beast or any other ally. Decent if you don't have anything competing for your bonus action like Fleet of Foot or Crossbow Expert.
SCAG also has Swashbuckler. But it's a lot more melee focused. Well, Rakish Audacity can add CHR to init to boost it further, if you have a good CHR, but considering how good you are at that already that doesn't seem compelling.
Lastly is Inquisitive from The UA Gothic Heroes. It has some ability to gain SA even without an ally, but you always have your beast and focus fire makes sense.
I went into this level of detail because none really jump out and yell "PICK ME". There are a number of solid choices, go for whatever fits your character the most.
As for an ASI, it's hard to go wrong with +2 DEX for your first. Bonus to hit, damage, AC, and initiative. Plus a common save, mostly against damage. I've mentioned Crossbow expert but that might be more worthwhile when you up your SA more to make sure you land at least one blow. Though if you fight favored enemies a lot maybe it's worth it now. Sharpshooter you probably won't want because it become counter-productive at higher levels: you don't want to miss and lose all your SA. It's other bonuses don't suck though (no disadvantage at long range plus ignoring 1/2 and 3/4 cover).
Other options, though more situational so probably for later.
- Lucky is always fun, and gains utility at later levels to help land SA and not lose it. Plus it's very good defensively, especially at avoiding crits.
- If you have an odd dex, any of the +1 dex half feats will raise your modifier and give you something.
- If you have a good CHR (min 13), Inspiring Leader can help you, your party, and your beast with temp HPs once a short rest.
- Mobile gives you more speed to kite, the ability to ignore difficult terrain on a round you use dash (and Ranger 8 would give you Dash as a bonus action), and the ability to disengage and move if you make a melee attack if some comes up to you.
- Alert feat means you will always go first - advantage, high dex and +5 from alert. But you probably already go first almost all of the time anyway.
- Healer can let you use a Healer's Kit to heal wounds (each creature only once per rest), and the Thief subclass lets you use an item as a bonus action so you can do it in addition to your normal stuff. But it doesn't scale.