I'm not too nuts about optimizing the character (obviously...), but at the same time I don't want a useless character at the same time.
You'll have to be more specific as to what you think a "useless" character/Warlock might be.
So my question is... is it just hopeless to have a melee-focused warlock who is not a hexblade or blade pact?
It's not hopeless, just limited.
If you go this route realize that you are forever only getting 1 attack/round. And while stabbing things you won't be able to command your familiar to attack & that it'll (probably) interfere with casting. You're also only proficient in light armor & simple weapons class-wise.
As you stated dex based, sword wielding, with fey patron, am I correct in guessing you're thinking maybe Elf (possibly Drow) as your race?
Mechanically being a Drow would be the best elf choice giving you Dex+2 (for AC, initiative, pairs nicely with finesse & ranged weapons), Cha+1 (your spellcasting stat), 120' darkvision, & proficiency with Rapiers, short swords, & hand xbows (oh look: finesse & ranged weapons!) + additional limited cantrips/spells from Drow Magic. You would have to deal with that pesky sunlight sensitivity though.... as well as making sure Drow are allowed as player options.
Alternately you could do a mountain dwarf for the Str+2 & medium armor prof & try and talk the DM into letting you switch out one of the weapon prof for a sword. Of course you still want your dex/cha & now your looking for more str.
If you aren't an elf of some sort, then you'll need to figure out a way to get proficiency with whatever sword you aim to use.
You could do that as a variant Human & using your feat. Or you could take at least 1 lv of a class that provides it to start with, say: Fighter 1, Warlock x. You will need to qualify stat wise for multi-classing though.
Spell wise you're also
really limited as a warlock. In # of spells known, spell selection, & most importantly in spell slots. For most of this characters career you're only going to have 2-3 spell slots per short rest.
Yes, I know. You look at the Wizard & think "they don't have that many more slots than I do since mine come back after a short rest" But those short rests? Access to them is 100% dependent upon the game you find yourself in.
This makes choosing what spells you know very important. Choose well.
Do you think it would be doable by taking a few levels in fighter first?
It would be very doable. And probably even the smart choice mechanically as you'd start with armor, shield & weapon proficiencies + a fighting style as you want to be in melee.
You could even combine that with being an Elf/Drow....
Of course the very best combo of race/class/etc? Is whatever let's you best express the c
haracter that you're envisioning. Even if that's not the optimal mechanical choice. Whenever in doubt, go with the
character vs just optimal mechanics.