Jeremiah sets out a list of nations starting with Judah to experience Jehovah's judgement whether it is of a conquering order or otherwise is not made plain in the prophecy for it begins with Judah and then successively lists and describes the fate of each nation.
Starting in verse 17, the list of nations is just an enumeration of all the nations that will serve Babylon, in one form or another, for 70 years. Jeremiah just got done stating that "these nations" will serve, and for emphasis, he starts to list them out, getting more general toward the end of the list. It's a way of underscoring the clear grammar of verse 11 - there are a plural number of nations serving Babylon. The listing doesn't imply conquering order at all - that's why you reach for the unusual translation of verse 29.
Going to biblegateway.com again, enumerating v29 in every translation they have, it becomes pretty clear. All of them, except one, render the verse and some derivation of "beginning to bring". The CEV uses "first", but it doesn't surprise me that you find at least one on your side, especially among the versions that don't mind wondering from the literal language:
KJ21
For lo, I begin to bring evil..
ASV
For, lo, I begin to work evil...
AMP
For behold, I am beginning to work disaster ...
AMPC
For behold, I am beginning to work evil ...
BRG
For, lo, I begin to bring evil ...
CSB
For I am already bringing disaster...
CEB
Look! I’m bringing disaster ...
CJB
For, look! — if I am bringing disaster ...
CEV
Starting with my own city of Jerusalem, everyone on earth will suffer from war.
DARBY
For behold, I begin to bring evil ...
DRA
For behold I begin to bring evil ...
ERV
I am already making these bad things happen ...
EHV
See, I am beginning to bring disaster ...
ESV
For behold, I begin to work disaster ...
ESVUK
For behold, I begin to work disaster ...
EXB
Look! I am ·already bringing [beginning to bring] ·disaster ...
GNV
For lo, I begin to plague the city ...
GW
I am going to bring disaster ...
GNT
I will begin my work of destruction ...
HCSB
For I am already bringing disaster ...
ICB
I am already bringing disaster ...
ISV
Look, I’m beginning to bring disaster ...
JUB
For, Behold, I begin to bring evil ...
KJV
For, lo, I begin to bring evil ...
AKJV
For, lo, I begin to bring evil ...
LSB
For behold, I am beginning to bring ...
LEB
For look, on the city that is called by my name I am beginning to inflict harm ...
TLB
I have begun to punish ...
MSG
“‘Prepare for the worst! I’m starting off the catastrophe in the city ...
MEV
For I am starting to bring calamity ...
NOG
I am going to bring disaster ...
NABRE
Now that I am inflicting evil on this city ...
NASB
For behold, I am beginning to inflict disaster ...
NASB1995
For behold, I am beginning to work calamity ...
NCB
Behold, I am beginning to bring disaster ...
NCV
Look! I am already bringing disaster ...
NET
For take note, I am already beginning to bring disaster ...
NIRV
I am beginning to bring trouble ...
NIV
See, I am beginning to bring disaster ...
NIVUK
See, I am beginning to bring disaster ...
NKJV
For behold, I begin to bring calamity ...
NLV
See, I am beginning to make trouble ...
NLT
I have begun to punish Jerusalem ...
NRSVA
See, I am beginning to bring disaster ...
NRSVACE
See, I am beginning to bring disaster ...
NRSVCE
See, I am beginning to bring disaster ...
NRSVUE
See, I am beginning to bring disaster ...
OJB
For, hinei, I begin to bring ra’ah on the Ir ...
RSV
For behold, I begin to work evil ...
RSVCE
For behold, I begin to work evil ...
TLV
See, I am beginning to bring evil ...
VOICE
Do you think I will bring disaster ...
WEB
For, behold, I begin to work evil ...
WYC
for lo! in the city in which my name is called to help, I begin to torment ...
YLT
For lo, in the city over which My name is called, I am beginning to do evil ...
The rendering of "beginning to bring" makes much more sense. It aligns perfectly with v18 - "as it is this day". And it doesn't try to turn "nations" into a singular, and "serve" into "desolation".
It also makes physical sense - as your proposal would require Babylonian armies marching over long distances, for months, without attacking anyone, leaving their worst enemy, Assyria, standing while Babylon is unguarded.