Some other possible suggestions.
In the late 80's or early 90's I went on an engineering field trip from Laramie to CO. One of the stops that day was at SERI. Solar energy research institute in Golden CO. One of the things they were working on was portable mirrors with adjustable focus. It was snap together pipe frame work that they put in a fairly tight mylar bag. The frame stretched the bag gently. Sort of like a giant drum. They then applied a light vacuum to the bag. By adjusting the vacuum it gave them a large variable focus parabolic mirror as the plastic stretched in its elastic limit. It eventually turned out they were too fragile for long term use. But this might be a faster way to prototype to what you want. Get your base measurements off something like this so you only need one base prototype. They are now known as NREL.(National renewable energy labratory)
NREL
I had been looking at something similar. One of the things I looked at was cast material. American Science and Surplus used to have bulk boxes of the plaster impregnated cloth for casts fairly cheap. Don't know if they still do.
Another suggestion would be to look for old satellite dishes. My father as a side gig sold satellite dishes in the late 70's. They came in household sizes up to 12 feet in diameter. 8 and 10 were common and as the electronics got better they shrunk down to 5 and 6. There were stamped metal ones and and rigid fiber glass ones. Big thing is they were designed to hold up out in the weather over the long haul. Would still need to be mirrored. The stamped metal ones were painted black to reduce what the reflected out to the LNA so they didn't cook the electronics. So if you could find one of them in good shape you might be able to simply polish the paint off,
Then another mirror option. Read down the list here. One of these was an outdoor rated mirror surface with something like 95% reflectivity and something like a 25 year warranty. Lighter weight and non breakable and no surface to peel off the back from weathering.
Mirror option It is one my wishful thinking list for several potential projects. A tad pricey but might be worth it for long term projects. As 2 of mine are over head the added safety of not being breakable would be worth it probably.
Suggest looking at the information on metal sand casting as they have a number of fairly cheap and safe ways to glue sand together for casting. Adding water glass aka sodium silicate is one. Wood flour is another and there were probably a dozen others. You will want something cheap and low waste while you make your positives to cast off from.
Lesson learned the hard way on paper mache done with flour if it has any moisture exposure(even just high humidity) it can draw bugs and make a mess over the longer haul. Given what I know now if I expected to want it longer term I would borate treat it in some form. Probably mixed with the water right to begin with. But you can also get the powder to mix with water so you can spray lumber with it too for termite and fire resistance so that would be my second option. Available at any good lumber yard.
Now I had been dreaming on other things and wanting to play with aircrete too. My thinking had been to do a skim coat of concrete, fiber, sand and concrete dye over the mold and then while still wet to pour it full of aircrete mix with fiber. That would likely work for you too giving a lighter weight rigid parabolic shape that should be durable. Have never gotten around to testing as other projects have priority.
As for ferrocrete many of the things I want to do there would be thin giving a high risk of it spalling over the time so the wishful thinking to try there is Basalt fiber instead of steel wire for the form material to put the concrete on.