Hi
I'll try to give some additional help on this, however I'd strongly recommend first reading Vaclaf's LF TG builds for both I&B and CM-patch, this is where you'll likely find most of the answers you're seeking for about this build, if not done already. Mechanix X-E for Ice&Blood / Mechanix Omega-TX for CM-patch
You'll also find his videos on his YT channel here.
The game allows you pretty much to do anything you want with your build, be it that you use only one or more aspects. As desm said above, you can play purely with Lost Fusion, it'll work, but you can also add Devout guardian mainly for it's energy shield (as you can see in Vaclaf's builds), or Source warden for debuffs (For example you can use Fiery Ember to reduce your opponent's armor against Fire damage before using Flamethrower, and even their attack speed if using the right mod) and eventually damage, it's up to you.
The answer isn't only for this question in particular - everything you do will get better as you level up, as you'll slowly increase your skills and get better gear. Most builds at start of the game, have horrible regen time and damage on CAs and you need to kill with your weapon until some time when you get a good balance between regen time and damage.
Also worth mentionning it gets much better when you have a character with Bargaining to support with +All skills jewelry and some other useful goodies - Blacksmith skill helps as well but as I understand you're already using it, which is great.
Skills can be cast quicker if you increase your casting speed by either spending points on corresponding aspect Lore skill or use +xx% cast times bonus on items (mostly on rings), keep in mind the maximum is 150% cast speed.
I think like all magic-based CAs the flame thrower has 100% hit chance, it's use is just tricky because of it's bad range, quoting Vaclaf on this one :
You should always use the mount that's dedicated to your main aspect as it gives a regen time reduction bonus.
For you weapon of choice I'll once again redirect you to Vaclaf's builds, and add that you don't need to pick any weapon skill for your build.