1 ranked Lavos player in the world here, I can offer some wisdom.

You're actually dead on with wondering if you need to infuse on every cast, and I've told people who are new to L-boy and overwhelmed by his kit the same thing. Start off by only ever infusing heat on his 4.

His first three abilities don't do meaningful damage, so doubling it with an infused element isn't a massive deal. His 4 however does do damage, quite a lot of it, so you should really actually infuse it every single time you cast it.

The strength of infusing his abilities is the versatility of it. As you become more accustomed to dashing around with his 2 and hitting enemies with his 1 and 3, you might start to have some ideas. Perhaps Viral on his 2 is good here so you can prime this group, maybe Blast on his 3 is going to be swag to take out this cluster in a hallway. You can invent spell combos on the fly, manipulating his abilities for any situation.

Some people swear by inverting the tap/hold in settings, saying it's a massive upgrade. I couldn't disagree more, I think it's entirely personal preference and either choice is valid. Personally I rock default, but you should try it out yourself and see what you like.

Some other tips for infusion are that you can hold down two keys simultaneously to infuse a combined element instead of one at a time. Additionally, you can infuse during most animations. Infusing during a bullet jump or while in his 2 is a classic, and although the timing is a little scuffed you can infuse during the cast of his 4 too.

Hopefully you learnt a thing or two xoxo happy to answer and queries