There's two main things to keep in mind here:
1) The game isn't hard enough to punish exploring different non-optimal concepts. A lot of folks don't even realize the power of things like Hold Person, because while it's objectively superior to just dumping damage, dumping damage is enough to do the job even in honor mode. I intend to eventually do a jack of all trades honor mode solo run, because frankly the game isn't hard enough for that to be unattainable. So no worries if you want multiple classes even just for lore or feelings, it shouldn't prevent you from clearing end game content, and if it is preventing you from doing so, you can revisit your tactics, prep, and approach to combats and find massive opportunities for improvement far before your build needs to change.
2) For any build, when it comes to adding a level of something else vs a level of your current class, the question is always simply this:
What am I gaining, and what am I giving up?
For most spellcasters, for example, I like to splash a level of Wizard for at least one if not multiple casters. This is because with the way Larian implemented and interpreted Wizards, one level of them is enough to provide access to all spells available to a Wizard of their total spellcaster level. So a Bard 11 / Wizard 1 is giving up a feat, and a spell known for Bard. That feat could be an ASI for Charisma, so they're potentially giving up +1 to their spell save DC and spell attack modifier, or it could be something else of course. In return, they gain the ability to know all Wizard spells of 1st through 6th level, the ability to cast any and all of them out of combat, and an additional amount of prepared spells in combat equal to 1+Int Mod. Generally I find that tradeoff to be advantageous.
Bard 8/Sorc 3/Wiz 1 gains +1 total spells known vs Bard 11 / Wiz 1, far more cantrips, and more importantly gains metamagic including possibly twin spell and quicken spell, letting them "go nova" via the expenditure of their long rest resources more effectively, in exchange for losing out on 5th and 6th level bard spells, magical secrets, expertise, better bardic inspiration, and some HP. Is that worth it? Maybe maybe not, depends on your party, strategies, and playstyle including but not limited to how often you long rest. But that's the kind of evaluation you'll want to do. So I ask you - when adding warlock, what exactly do you hope to gain from it, and how does the value of that compare to what you'll miss out on?
