Fire Gem? Straight Sword?

Just got one. I'm using Astora Straight Sword +3 currently. I have a Lothric Knight sword but that needs 18 Dex. So, should I even bother leveling enough to use the Lothric sword and craft that into fire, use my Astora sword till I can't level it anymore (+x) and then turn it into fire, or wait cause I will be getting a better straight sword soon? Pls minimal spoilers just looking for some guidance!