Here is the broader context of Proverbs 8:27 from the OP's selected NKJV:
22 “The Lord possessed me at the beginning of His way, Before His works of old.
23 I have been established from everlasting, From the beginning, before there was ever an earth.
24 When there were no depths I was brought forth, When there were no fountains abounding with water.
25 Before the mountains were settled, Before the hills, I was brought forth;
26 While as yet He had not made the earth or the fields, Or the primal dust of the world.
27 When He prepared the heavens, I was there, When He drew a circle on the face of the deep,
28 When He established the clouds above, When He strengthened the fountains of the deep,
29 When He assigned to the sea its limit, So that the waters would not transgress His command, When He marked out the foundations of the earth,
30 Then I was beside Him as a master craftsman; And I was daily His delight, Rejoicing always before Him,
31 Rejoicing in His inhabited world, And my delight was with the sons of men.
The OP has assumed without any evidence to support it that "the deep" is something other than the earth, yet everything subsequent to verse 27 is discussing features of the earth. For example, verse 28 mentions "the clouds above"... above what? The deep.
Let's see what Genesis 1 says, again from the NKJV:
1 In the beginning God created the heavens and the earth.
2 The earth was without form, and void; and darkness was on the face of the deep. And the Spirit of God was hovering over the face of the waters.
There is no clear statement, "this is that", but from the wording, we can rationally derive that "the deep" is the primordial ocean. Nothing in the wording of either passage requires it to be flat (or spherical, for that matter).