I'm in a group that's playing through Tomb of Annihilation. Our DM told us to have a backup character ready because it's very likely someone may die. I'm currently playing a rogue who was kind of forced to become more of a frontline fighter (swashbuckler 5, battle master 4) because that's what the party needed, and, it turns out, I'm really good at this role. I bring this up because a common question asked in bladesinger threads is, "how do you plan on playing the bladesinger," and I'm loving this mobile, hit-and-run fighting style. As a rogue, I can basically only stab in combat. I wanted to play something that can still swing a sword but who is also more versatile.

I landed on either eldritch knight or bladesinger and ultimately decided to go the BS route for my backup character because I'd have two additional spell levels at the cost of about 20 HP. I'm worried we may find ourselves up against a lich, so I wanted dispel and counterspell (our party has made it this far without a wizard). I don't expect my play style to differ too much from my rogue except that I'm missing out on sneak attack and free disengage from rogue/swashbuckler (and I'm SO tempted to take mobility over an ASI...).

My bladesinger will start as fighter 1, wizard 8. I opted for the extra ASI/feat over Action Surge at this point, but the final few levels of this campaign are all going into fighter. I'm perfectly happy with the BS's spell progression being halted because I still have twice the spell levels of an EK.

I considered rogue instead of fighter, but, for flavor and roleplay, fighter levels make more sense for this character concept: a half-elf who was an accomplished swordsman in his own right who is fascinated by elven history and bladesingers specifically (cloistered scholar background). He is essentially a self-taught bladesinger through study and research with some minor magical training through his contacts that he combined with his skill in swordplay. For this concept, the first level as fighter is mandatory.

Every guide online says the first level fighter dip is GREAT for a melee-oriented bladesinger, and, honestly, I'm not buying it. I really just don't understand why this is any better than a straight bladesinger (aside for roleplaying the character in my head).

Taking the first level of fighter gives me CON saves proficiency, and I understand the importance of this for a bladesinger, BUT I lose WIS proficiency which, upon more reading and research, is not something I want to give up so easily since CON and WIS saves seem to be, more or less, equally important. This means I'm burning a feat on Resilience (WIS). Absent the level of fighter, I feel like Resilience (CON) would have been better for a straight BS because that 1 to CON from the feat could potentially increase HP.

If the main benefit is the CON proficiency, and I feel compelled to take a Resilience feat one way or another, then the main benefit of starting as fighter isn't a benefit at all. If the majority online recommends a single fighter level dip for a melee-focussed BS, are they all in silent agreement that they want no additional protection against spells that call for a WIS save? Because a BS is not going to have a high WIS. Am I putting too much importance on a save that could potentially save me from being disabled for an entire battle?

The other, starting at level 1 benefits of fighter are equipment (useless for BS) and skill proficiencies proficiencies (less useful for half-elf) and 4 starting HP (only useful at level 1, not at level 9). The other, general benefits of a level of fighter are a fighting style and Second Wind. Second Wind is decent. It's essentially a free potion (but an extra level in BS is an extra spell slot that could be used on False Life if needed). The fighting style (dueling) is good, though, since I'm not going to make it to level 14 for the BS ability to add my INT modifier to my attacks, but is this single feature so good that a single level of fighter is so often recommended for BS builds?

The best benefit of fighter that I see is taking the second level for action surge. That's great. But most BS guides say just one level of fighter is all a BS needs to be more effective as a melee build.

Why? What am I missing to not understand the usefulness of a single fighter level? I'm taking it (followed by more) because it fits my character's background, but I don't see the point when it comes to in-game mechanics and making my character more effective than they would be as a straight classes wizard.

If it's useful for discussion, my stat spread (after adding the 1 from Resilience (WIS) and 2 DEX from an ASI are: 8, 18, 14, 16, 12, 10. HP is 60. Increasing my DEX to 18 was nice, but an additional 18 HP from Tough would have been nice, too, or taking the Mobile feat so I could be better suited to hit-and-run...