Mystic Bladesinger Multiclass Advice

[Innate]

Our DM has agreed that I can play the Mystic class and that I can multiclass as well. I'd appreciate any advice that people have about what would be the most optimal build (just in terms of leveling) for Mystic and Bladesinger at level 20. I thought about multiclassing with Eldritch Knight but I think that I prefer Bladesinger. Would it be a 1-3 level dip in either class, or some type of more even split?

I understand that Mystic's abilities sharply taper off about 9th or 10th level, but what combination of the two classes likely results in the most optimized character at mid levels and at 20th level? I like both classes equally so it's not really an issue of preference for either class as the primary. Thanks in advance.

[Innate]

In terms of Orders for Mystic, I also wasn't sure what to pick. A number of people have commented that Immortal would help out a bladesinger build, but Nomad seems like it has a lot of great features that would have a lot of in-game utility.

[Jerrykhor]

Wow your DM allows you to multiclass Mystic with other class, you can make some really powerful combo with that. My first thought is stacking AC. With lv 3 Mystic, you can take 4 Disciplines if you are not Soulknife. I'd recommend Immortal for sheer toughness if you plan to be in melee, which you should since you invest in all that AC. The best case scenario is if you can max out your Dex, Con and Int. If you can, the AC you can reach is very high. If you can't, then take Mastery of Force for Inertial Armour, That will start you off at 19AC if you have 20 Dex.

Mystic Disciplines:
Bestial Form (+2 AC from Tough Hide)
Mastery of Force (19AC)
Mastery of Wood and Earth (+1 AC from focus)
Mastery of Air (for Cloak of Air)

Wizard
Bladesong (+5 AC if 20int)
Shield spell (+5 AC)

You can get a total of 32 AC from this, 27AC without Shield. On top of that, Cloak of Air will make all attacks have disadvantage against you, and you have temp HP from Psionic Resilience. All this from just Mystic 3 Wizard 2.

[Drascin]

Do note you cannot stack the mastery of force mage armor with the Immortal extra AC calculation. They both alter base AC. So you have to remember to pick and choose, and often it might not be worth it to pick an entire discipline just to get a couple more AC points, because well, people rarely start out with 20 Int and 20 Dex.

In any case, yes, as you have detected, OP, Mystic has extremely frontloaded features and then sharply tapers off (this is part of why DMs rarely let you just multiclass into it, because otherwise dipping Mystic 3 for a whole batch of goodies becomes too tempting for way too many classes). It's basically the Warlock problem only more so, where you get a ton of stuff for your first five levels.

So my general suggestion would be to not try to minmax this, but if you absolutely need to minmax this, you should only take Mystic up to about 5 or 6 (if your subclass feature looks enticing to you) and then just go singer the rest of the way. Immortal seems the most applicable subclass, though as a bladesigner you might want to consider if you want to bother with the Immortal alternate AC route or not. Both the extra temp HP and the save yourself from getting murked Immortal features are very useful stuff, too.

For disciplines, Bestial Form is always useful, as it provides a lot of much needed mobility that a melee fighter can always use, letting you take the melee fighting to underwater and to the air with relatively little investment, as well as a bit of extra AC (do take a moment to consider how you're going to flavor your elegant rapier-wielding elf mage growing a whole ass crab's carapace, aesthetically!). Pisonic Weapon fits the bladesinger image very well and gives you both a pseudo-smite to go nova on a guy (keep in mind that lethal strike takes your bonus action to use and is declared before you attack, so unlike the paladin's smite it can be wasted - save it for when you have advantage!) and a way to get through BPS resistances with minimal fuss. Iron Durability is a definite winner, as it has a focus that gives +1 AC plus a pseudo-Shield. Consider also, depending on campaign and DM proclivities, Intellect Fortress - being an Immortal/Bladesigner is going to need you to focus very hard on your Dex/Con/Int, which makes it very likely you will have to near-dump Cha and Wis, and in some campaigns having no way to shore up a terrible Wis or Cha save can easily be a game ender.

In general, be very careful about assuming Christmas land perfect scenarios. Mystic power points run out very fast and you have no way to recover them, and your bonus action slot every turn is going to be very crowded, as every goddamn thing you will want to do as a Bladesinger/Immortal requires your bonus action and you have only one of those each turn, so it can take you a few turns to rev up to maximum combat readiness.