The bass just moves
^4 ^5 ^1 or
FA SOL DO or
Bb C F. I think the ideal thing is for
^4 ^5 to be an upward step rather than a descending 7th.
But, the upper voices need to be reviewed.
According to Piston's Harmony
ii V (including
ii6 V) is different than other descending fifth root progressions. Instead of the upper voices moving up by step to the next closest chord tones they jump down by thirds...
...Piston doesn't say, but I think the reason for this motion is to get the top part to move down to the leading tone and then up to the tonic (highlighted green.) That motion of a sixth
Bb G to a third
C E - or a chord of the sixth to a root position chord - is certainly a counterpoint fundamental.
If we add the
V - and keep to 4 voices, we have to drop a tone. The chord's 5th is usually the one to omit. So, we drop the 5th of
V, add the
7, and get
It seems to me we could have not doubled the root of
V and then used the 5th with the voicing from bass to soprano
C Bb E G...
...that would result in the leading tone moving down to the dominant, the 5th of
I. Apparently that 'frustrated' leading tone motion (highlighted red) was not ideal and is the reason for Piston's advice.
Piston didn't specifically address
ii7. It seems to me the important thing is the
7 added to
ii is the tonic! In terms of smooth voice leading
^1 ^7 ^1 seems the smoothest motion for the top line...
...leaving all the other tones in place we move the tenor from
G in lieu of
G no longer being in the soprano.
Because the main topic here is voice leading I thought it good to inventory the basic lines for each of the upper parts in the various examples. Of course the bass for all of them is
^4 ^5 ^1 or
FA SOL DO. The upper parts are...
^2 ^7 ^1 or RE TI DO
^6 ^5 ^5 or LA SOL SOL
^4 ^2 ^3 or FA RE MI
^4 ^4 ^3 or FA FA MI
^1 ^7 ^1 or DO TI DO
^2 ^4 ^3 or RE FA MI
The various upper parts move in smooth steps or thirds that resolve by a step back in the opposite direction. And we have the tendency tones
TI moving respectively to
The upper voices should all work in various inversions and open positions, except for the 'frustrated' leading tone example which is normally only acceptable when it happens in an inner voice.
With the exception of the questionable 'frustrated' leading tone example I think the rest of this is solid.
...what other way could I rewrite the chords to make the Bass move smoothly?
Schoenberg gave the advice to "always follow the bass!"
His point being that the bass generates the harmony.
From that point of view, I suggest re-framing your question like "how do I write my upper voices to best harmonize with a good bass?"
In other words make sure your bass is good first, then harmonize the upper parts and change the upper parts as needed. Don't mess around with a good bass! Imagine a house metaphor. You can remodel the rooms of your house, but you really don't want to re-do the foundation!