According to Isaiah, Yahweh will shave "pubic hair" (רֶגֶל (regel) with a razor?

In that day the Lord will shave with a razor that is hired beyond the River—with the king of Assyria—the head and the hair of the feet, and it will sweep away the beard also. (Isaiah 7:20)

Talmudic commentary suggests רֶגֶל (regel),means pubic hair or the private part area

The Gemara continues its challenge. Come and hear another verse: “He had neither dressed his feet [raglav], nor trimmed his beard” (II Samuel 19:25). The phrase “dressed his feet [raglav]” is referring to treating his pubic hair, implying that even the area around the thigh is referred to as regel. The Gemara answers: This is a euphemism. The Gemara attempts another challenge: Come and hear from another verse: “And Saul went in to cover his feet [raglav]” (I Samuel 24:3), meaning: To urinate, implying that regel refers even to the thighs. The Gemara answers: This is also a euphemism. (Yevamot 103a:15)

Yahweh would shave pubic hair רֶגֶל regel,? Seriously! Let's not get confused, according to 1 Samuel 24:3 Saul went to "COVER HIS FEET" (רֶגֶל regel, what does that mean? Well, according to Rashi medieval Tanak scholar "to cover his feet" means : To stool (Call of nature, having to go toilet to get rid of waste! i.e. taking a crap, having a S**T (Emphasis added)

Meaning hair from front and back will be shaved by a razor.

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Isaiah historical error. Two versions of Isaiah by different authors.      

1st and 2nd Isaiah

 

Start with Isaiah 1:1 is close to history chapter 6 speaks of the vison in the temple speaking to God who is above him and the hem of God is in the temple. He hears God speaking to others who to send, Isaiah wants to be that man to be sent. Isaiah volunteers his mission commission. He does not want the people of Judea, Judeans to be doomed. Chapter 10 the enemy of that punishment will be Assyrian. The Assyrian will rule of Judah and  conquest,  military conquest over Judea and Jerusalem as a punishment. Each of these details, temple, Hezekiah, Assyria as a foreign empire fit perfect with 8 century  date with Isaiah history

 

Compare to chapter 40. now the same god is speaking good of Jerusalem. And the old enemy Assyria is no longer there nor is Babylonia The new enemy is Cyrus of Persia. Isaiah mentions Cyrus many times, calls him the anointed one his messiah, his Shepard.  The message Isaiah brings is, the lord aroused Cyrus in order so he can rebuild my city and set my exiles free. In the first chapter Assyria is a weapon, and in chapter 40 Cyrus of Persia is used to subdue other nations and to rebuild Jerusalem. This is a different historical context. These discrepancies in date, tone and style

 

 

The OT Prophet Isaiah lived in the 8th-century BC. Cyrus, the Emperor of Persia, lived well over one hundred years later: Cyrus (580-529 BC) was the first Achaemenid Emperor. Yet The Book of Isaiah in Chapters 44 and 45 speaks of Cyrus in no uncertain terms:

 

 

The earliest manuscript we have of the Book of Isaiah is The Great Isaiah Scroll:

 

The Great Isaiah Scroll (1QIsaa) is one of the original seven Dead Sea Scrolls discovered in Qumran in 1947. It is the largest (734 cm) and best preserved of all the biblical scrolls, and the only one that is almost complete. The 54 columns contain all 66 chapters of the Hebrew version of the biblical Book of Isaiah. Dating from ca. 125 BCE, it is also one of the oldest of the Dead Sea Scrolls, some one thousand years older than the oldest manuscripts of the Hebrew Bible known to us before the scrolls' discovery.

So our oldest extant copy of Isaiah dates to a much later period than that of Cyrus.

 

More directly regarding the question at hand:

 

Supra:

 

Modern scholarship considers the Book of Isaiah to be an anthology, the two principal compositions of which are the Book of Isaiah proper (chapters 1-39, with some exceptions), containing the words of the prophet Isaiah himself, dating from the time of the First Temple, around 700 BCE, and Second Isaiah (Deutero-Isaiah, chapters 40-66), comprising the words of an anonymous prophet, who lived some one hundred and fifty years later, around the time of the Babylonian exile and the restoration of the Temple in the Persian Period. By the time our Isaiah Scroll was copied (the last third of the second century BCE), the book was already regarded as a single composition. If so, no miracles or prophecies are required to explain the mention of Cyrus in the Book of Isaiah: Chapter 45, where his name is mentioned, was originally written during the time of Cyrus's rule.

 

 

 While Isa 1-39 has the time of the downfall of Samaria in mind, that is, in the 8th century BC, the author of Isa 40-55 already expects the end of the Babylonian kingdom (Isa 43,14; 46-47) and the rise of the Persian Cyrus (Isa 44,26-27 and others