hey thanks for pointing out those 2 things about the bladeborn, im trying to actually think of how you could get some form of armor and i want your opinion on what sounds better
-Reinforcements that take smithing tools to attach and cost the same as the armor they give the equivalent ac to, does kinda not make sense what the different of half plate and plate is on a sword though
-any + enchantment on whatever enchantment you currently have also adds to armor class as long as you are not puppeteering anything, does kinda make it hard to be any heavy armor focused class though
Some potential solutions re: Bladeborns and armor:
1. Similar to barding for animals, armor can be modified to suit you. Could potentially be flavored as reinforcement or a special scabbard, though you're right that it kinda falls apart if you ask what the difference between different armor types is. Not to mention a good number of weapons wouldn't use scabbards, like bows. A "work with your DM to determine the presentation of this feature" wouldn't hurt.
2. Flat AC from your race, similar to Tortle, makes a decent amount of sense, given that most weapons are primarily made of metal and designed to clash with other weapons. Tortle isn't exactly a shining star of balance, but that's more because of how it interacts with casters, which the Bladeborn isn't synergistic with (kinda hard to hold a focus without hands). Being unable to benefit from most magic armors designed for martials is kind of a bummer, though.
3. Natural armor, scaling with either Dex or Con (as with Lizardfolk), could work - forces you to at least invest a bit for the benefits of AC. Similar problems with magic items remain, however. Actually, given that youre a sentient weapon, you also have similar problems with magic weapons as well - other people could be getting handed Flametongues and Frostbrands and you're stuck just being a normal sword. Perhaps the ability to absorb/transform into other magic items that you attune to in order to gain their benefits? Probably want to be careful with how that works, but that's not an uncommon trope for cursed weapons. Also, trying to possess a weapon that's already sentient might run into some. Interesting issues.
4. Possibly in conjunction with any of the above options, an AC bonus based on how many hands it takes to wield your chosen weapon? All else being equal, you're basically forced to go for a two-handed weapon since you (presumably) can't dual wield or use a shield without hands, which forces you to go for a Strength build. If you want to be a flying rapier, getting a +2 AC to mimic the benefits of a shield would be nice. Though then you can do something cheesy like Great Weapon Fighting style on a longsword, effectively letting you benefit from both a two-handed weapon and a one-handed one simultaneously... Might need a stipulation that you can only wield yourself in one "hand" to get that bonus then?
also i didnt think of this while writing the feedback on the post but. What would happen if someone used the disarm maneuver on you.