Although I've never taken it myself, spiro is a synthetic hormone, and all synthetic hormones seem to become toxic if you take them continuously over a long period of time. I think there's likely to be a particularly high risk of adverse side effects with spiro, because its main action is on mineralocorticoid receptors, and the antiandrogen effect is just a secondary effect. So you're mucking around with your mineralocorticoid receptors for no reason at all, other than because trans healthcare workers were looking for an antiandrogen, and here was this relatively inexpensive and readily available blood pressure drug which happens to have an antiandrogen effect in addition to its main effect of lowering blood pressure.
Estradiol on its own does block T production, and apparently high enough blood E levels can achieve T suppression in most people without the need for any additional antiandrogen. From what I've heard, the only reason antiandrogens are needed is because the current standards of care aim for too low an E level to reliably suppress T, purely because of several past disasters in which doctors killed large numbers of women with synthetic estrogens. This has led to estrogens gaining an undeserved reputation for being dangerous, when actually it's synthetic hormones that are dangerous!