Hello, I am new to the forum, but have been reading this entry for 4 months to help me create my character for a game we started 2 months ago. I originally wanted to create a Fighter with a splash of Bard, but ended up doing the opposite. I think it works well but I have to agree that there are some other multi-class splashes that probably work better to replace another dedicated class entirely. Here are my thoughts:
Probably the easiest is a 1 level dip into Rogue (after at least 1 Bard level) to gain Sneak Attack, Expertise, Thieves' Cant and proficiency in Thieves' Tools if you don't have it from your background (like Charlatan to gain additional proficiencies in Disguise & Forgery Kits). Assuming you take a Half Elf and spread your skills and Expertise wisely, you have most of most of the bases covered for the Rouge's roll with the bonus of additional spell support, especially healing when things get tough.
Speaking of healing, a 1 level dip into Cleric of Life (again, after at least 1 Bard level) can eliminate the need for a Cleric's healing functionality, plus you get proficiency in Shield and Medium Armor if you go Lore Bard. If you have a Paladin in your party, you have the turning undead and other cleric functions covered which frees you up to also lend help in skill checks and broader selection of spells.
A 2 level dip into Warlock (after Bard 1) to get you Agonizing Blast (and Eldrich Spear, Eldrich Sight or Mask of Many Faces) can replace the need for a Warlock. Take the Human Variant with the Warcasting Feat, and if you wish build up STR instead of DEX and go Valor Bard for proficiency in Shield, Armor & Weapons and you are good both at range and up close giving you much more functionality and again, more spells from which to choose.
I know all those were already discussed, but I just think any of those choices are better for all around usefulness than the single class you are replacing. For my Bard, I wanted to be a Fighter with a 2 level Bard dip to add extra healing to our group. However, the Fighter in the group has the Polearm Master Feat, and kicks ass. Also, our Warlock dropped out, so I decided to instead advance fully in Bard after starting 1st level in Fighter. While it may not be as optimized the earlier suggestions, he is great in all around support with being "the face" of the party, skill proficiencies, additional spellcasting/healing and fighting both at range and melee.
There are a few benefits to starting Fighter 1st which include 2 extra HP, proficiency in Shield, Armor, Weapons & CON saving throws (i.e. Concentration spells), 2nd Wind for a quick Bonus Action HP boost and most importantly, Fighting Style. A real argument can be made for Archery or Dueling, but I chose the Defense Fighting Style for the +1AC that helps both in melee combat and more importantly when using my longbow and without my shield's +2 AC. Because I have a 16 DEX, have studded leather & shield + the Defense Fighting Style, I have a better AC at 18 than the frontline fighter & cleric (at least until they get heavy armor). Plus, with light armor, I can sneak into a room to get surprise. And now that I have chosen College of Lore, that 1 level dip helps with ranged attacks (Longbow vs. Hand Crossbow) and AC that I wouldn't have received if I was a dedicated Bard.
I think of him as a replacement for an Eldrich Knight where I'm sacrificing some of the fighting ability for better spells. Or just in a general sense, if you have a dedicated Cleric & Rogue then a better single-classed College of Lore Bard where you are only sacrificing (if you even reach it) your crappy level 20 capstone achievement. There is more I would like to add on other observations I've witnessed while playing a Bard, but I can discuss them at a later time. Any comments are appreciated.