Gating bronze tier of tech behind finding a Nether Fortress is a bit much, given how it's a massive hassle to explore the Nether. It all boils down to whether or not you luck out on your portal being close enough to spot the Fortress.

The suggested alternative is using lava source block to heat up the cauldron, with the following catches:

• lava-heated caudron will not allow use of superheated recipes
• lava source can get converted to a stone/magma block after running N recipes or with random chance after completing each recipe

This should ensure that lava heating will remain a stop-gap measure to get small quantities of brass and the like. It can also be an incentive for the player to automate lava collection, or at least locate the mixing factory near a lava lake/lava sea, away from their base.

Ultimately, blaze burner will be superior in terms of usability and capabilities, but it will remain gated behind fiding a Nether Fortress.

So what you are saying is that brass tech is end-game, not mid-game?
Also, seedfinders are for lazy people. =p

actually, brass is mid-game resource. Nether entry point is somewhere between early and mid game, mainly because of how easy it is to get into the Nether (you only need 3-4 iron for that). by the time you'll start demanding brass-tier stuff, you'll probably get to the Nether and get some resources from it

I... you consider the nether endgame? What?

Superheated blaze burner recipes aren't even that important. And the lava source running out doesn't matter, it's being able to use the heated mixer so quickly that really matters more than anything. I don't think this suggestion would be too balanced. Besides. Fortresses aren't even that hard to find people. Just travel diagonally for a while and try to travel on open areas, you'll find a fortress eventually. This is a suggestion for lazy people, we don't need to add support for lazy people. Hell even dump your seed into a fortress finder.

lava can be found outside the Nether, in the overworld. pretty much enough so you can make some bunch of brass before you get to the Nether. where's the challenge in this? only to find bunch of lava pools and to get all the lava to your base?

my point there, is that lava is rather early-game resource, available after you get iron (or, sometimes, before you even start mining), and Nether fortress is a mid-game location, which is to be found and then looted. and if you'll give players cheaper alternate recipe that is available earlier, they will prefer this alternate recipe, despite being designed to allow unlucky players who can't get into the fortress for some reason (i.e. unlucky world generation)...

TL;DR: if you're making an alternate recipe, make sure to lock it somewhere near original recipe, to prevent everyone from skipping using this alternate recipe

Same can be said about you, Honhk. You still haven't given a single solid argument as to why the brass should be gated that late except "because it should be so".
However, upon taking a closer look at the quote-unquote tech tree... yeah. Brass appears to be actually an end-game resource, simply because chromatic alloy seems to be still in development in terms of having usage on par with other materials.
In which case this discussion becomes totally moot.

Just entering Nether is indeed easy and can be done as soon as you get access to iron, so tech being gated behind quartz/glowstone/soulsand is not much of an obstacle. I agree with that much.

But finding a fortress is much less so, given the abysmal visibility, broken terrain and abundance of hostile and herd-neutral mobs. Unless you know precisely where to go (via seedfinder or lucking out with the portal spawning nearby), exploring Nether without Elytra is a hassle. Compared to the Nether, navigating the End is easy.

For a comparison, imagine some game mechanic that only works when player is inside Woodland Mansion. And then someone says, "Hey, that's not a problem, it's in overworld! You can go there on day one if you want to!" Technically true. Practically - not so much, given how rare and dangerous mansions are.

I want to point out that lava-heating will require either a lot of manual labor on replacing the source block regularly, or a moderately complicated contraption to do so automatically. Where as blaze burner only requires a source of regular furnace fuel and a simple contraption to feed it. Add the need to transport large quantities of lava to the setup, and I think it's fairly clear that lava-heating is nowhere near good enough to render blaze burner obsolete. It'd play the same role as being able to mix andesite alloy by hand instead of using a mixer - an early inefficient technique, useful when starting up but superseded by later tech.

Finally, if lava heating needs to be made even less convenient to ensure the player has an incentive to advance to using blaze burners, it can be further nerfed by requiring the basin to be surrounded by the lava blocks instead. Having to maintain 4 (or 5) source blocks instead of one will increase the complexity of required automation and makes it harder to mass-produce brass that way.

the Nether is designed to be hard to explore. and brass recipe is designed to be locked behind Nether fortress. it would be too OP to give player access to brass before they even get to the Nether. TBH, fire/soul fire heating makes much more sense and sounds a lot less OP, compared to lava heating, even tho lava heating would be harder to automate.

You are just making excuses at this point. This is not comparable to andesite and iron nuggets, as you can't even make a basin without andesite alloys. Just learn how to dig and bridge and you evade 50% of the treacherous terrain of the nether. Not that hard to do. People do it all the time. So should you. Hostile mobs are not even that hard. The only mob that will ever make you want to kill yourself is magma cubes and that's in basalt deltas. Also you can very easily ignore fortresses and use the infinite lava provided from entering the nether, since by your logic, fortresses are SO HARD to find. And it's really easy to automate putting 5 buckets of lava or 4 around a basin. Just put some leaky mechanical pumps, that's the easiest thing to do. Sure, you'll need to use rotation in the nether to get brass, but seeing as you agree that fortresses are hard to find, we might as well replace blaze burners entirely and just move half of your automation to the nether. You can easily keep the chunks in the nether loaded by ejecting the items through portals.
Edit: You can even immediately get brass once you get the required materials without having to go to the nether by just dumping the copper and zinc through a portal.