Estout to produce "wide" regression output with "append" option

[edonavot]

I am running mixed effects regressions on data which is structured as follows: I have about 30 data sets, each one with about 100K observations and representing a sample from one year. I need to run 30 regressions (15 different dependent variables using two different analytical techniques) per data set and save the regression output. Critically, I need the output organized by year. I'm using the excellent estout package to achieve this. However, with 30^2 saved results, I will very quickly exceed my version of Stata's limits on saved results. The easiest solution I could find for this limit is to call -esttab- within each data year then appending the results for each consecutive year. Unfortunately, -esttab- appends new results "below" previous results, resulting in a very long column that will be very tedious to restructure after all results are compiled.

The below code is a simple "reprex" that will produce the structure of regression output I would like:

version 13.1
webuse productivity, clear

forval y = 1970/1986 {
    qui mixed gsp || region : if year == `y', mle
    eststo vardecomp_y`y', noesample

    qui mixed gsp private emp hwy water other unemp || region : if year == `y', cov(un) mle
    eststo mix_y`y', noesample
}

esttab mix_y* using mix.csv, replace nostar noparentheses plain cells("b(fmt(a6) nostar) se(par fmt(a6)) t(par fmt(a6))")
esttab vardecomp_y* using vardecomp.csv, replace nostar noparentheses plain cells("b(fmt(a6) nostar) se(par fmt(a6)) t(par fmt(a6))")

Unfortunately, the size of my data and limited computing power force me to implement code similar to the following, which produces the "long" column of regression outut:

forval y = 1970/1986 {
    if `y' == 1979 {
        loc replace_append replace
    }
    else {
        loc replace_append append
    }

    qui mixed gsp || region : if year == `y', mle
    eststo vardecomp, noesample
    esttab vardecomp using vardecomp.csv, `replace_append' nostar noparentheses plain cells("b(fmt(a6) nostar) se(par fmt(a6)) t(par fmt(a6))")

    qui mixed gsp private emp hwy water other unemp || region : if year == `y', cov(un) mle
    eststo mix, noesample
    esttab mix using mix.csv, `replace_append' nostar noparentheses plain cells("b(fmt(a6) nostar) se(par fmt(a6)) t(par fmt(a6))")
}

I would appreciate advice on any of the following directions:

1. Ideally, I would like to use -estout- in combination with the _append_ option but produce "wide" output, as in the first code chunk.
2. Or if you see a simple/easy way to restructure the "long" output into wide after all the append operations are complete, this would be equally good.

Or perhaps the creative and experienced folks on this list see a completely different solution? Thank you very much in advance!