The question requires understanding exactly what the word "mūla" ("root") is meant to mean in terms of its general usage (which I cannot answer at this current time and requires a contextual analysis of the texts). However, given "attachment" actually is suffering itself, the word "root" may not mean a "preceding cause" ("hetu") but be something much closer to the subject/thing.
Delight in feelings is attachment - MN 38
In short, suffering is the five aggregates attached to - SN 56.11
Conceiving 'self' is an arrow, disease, cancer - MN 140
Resolved on 'my self'... is suffering arising... SN 12.15
View of 'a being'... is suffering arising... SN 5.10
In SN 42.11, 'root' is used as a preceding cause; although 'chando' is not as exact as 'tanha':
For desire is the root of suffering. Chando hi mūlaṃ dukkhassa.
In MN 22, something that is essentially 'causeless', namely, ignorance, is cut off at its root:
Herein the monk has abandoned ignorance, has cut it off at the root..
Abandoned the round of re-births, leading to renewed existence; he has cut it off at the root...
Abandoned craving, has cut it off at the root...
Abandoned the five lower fetters, has cut them off at the root...
Abandoned the conceit of self, has cut it off at the root...
The contextual analysis becomes more clear with SN 15.9, which says:
Suppose a stick was tossed up in the air. Sometimes it’d fall on its bottom, sometimes the middle, and sometimes the top.
Seyyathāpi, bhikkhave, daṇḍo uparivehāsaṃ khitto sakimpi mūlena nipatati, sakimpi majjhena nipatati, sakimpi antena nipatati;
Therefore, as I suspected, the word "mūla" might be closer to the subject/thing itself than being a distant preceding cause. In SN 15.9, it is the "bottom/base" of a stick (rather than a more distant cause, such as tree, tool or carpenter that made the stick). This appears apparent in SN 4.24 which says:
“Having dug up entirely the root of sorrow, Guiltless, I meditate free from sorrow. Having cut off all greedy urge for existence, I
meditate taintless, O kinsman of the negligent!”
Again, Snp 4.14 is about a very close cause rather than a distant cause:
One should completely extract the root of proliferation and reckoning— The notion, “I am the thinker”.
AN 10.58 says:
Wholesome zeal (chanda) is the root of all dhamma practise.
This says without zeal (chanda), dhamma practises will be ineffective, similar to how a car cannot drive without fuel.
I think 'mūla' is a characteristic or cause of a thing that is very closely tied to the thing. Therefore, 'ignorance' is probably too distant a cause to be the 'mūla' of suffering because what makes suffering burn is the craving, attachment & egoism.
Therefore, the "root" or "base" of suffering is craving & delight. The "trunk" of suffering is "becoming & ego-birth". The "branches & leaves" of suffering are "ego-aging-death, sorrow, lamentation, pain, grief & despair".
"Ignorance" is the first cause/leader of all unskilful qualities, which includes attachment.
Avijjā, bhikkhave, pubbaṅgamā akusalānaṃ dhammānaṃ samāpattiyā..
Monks, ignorance is the leader/foreunner in the attainment of unskillful qualities...
Purimā, bhikkhave, koṭi na paññāyati avijjāya: ‘ito pubbe avijjā nāhosi, atha pacchā samabhavī’ti
Bhikkhus, this is said: ‘A first point of ignorance, bhikkhus, is not seen such that before this there was no ignorance and afterward it came into being.’