I’d never introduce balance controls it not only moves the soundstage but limits the information to the channel.
The problem isn’t the amp it’s the placement of your speakers.
Try a mono recording when testing this too.
With such an imbalance due to the room acoustics you really do need an amp with a balance control onboard or else move the speakers in the room so the imbalance is minimised. The NAIT 5i is purposely stripped down to a minimum so there’s no balance control and no easy way to introduce one either.
You ansolutely need a balance control. Don’t faff about trying to implement an external balance control. Trade in yiur amp for one with a balance control.
The Naim 172, 272 and SuperUniti all have an internal balance control set by using the remote handset, even though there’s no external control knob. (I believe the other Uniti products also do.)
The SU is an upgrade in respect of the Nait 5i, and they are fairly low cost on the secondhand market.
An alternative to changing the amp might be to swap your streamer for one that includes a balance control. There are streamers that offer this including, I understand, well regarded budget versions.
One reason I moved from an XS 2 to a SuperNait 2 was precisely because the SuperNait had a balance control. I loved the XS 2 but never regretted the move. I love having a balance control. Along with having a non-symmetrical room, I find that recordings sometimes differ in left-right balance as well so to me it is important to be able to correct for that.
If you want a Naim integrated amp, unfortunately the next step up the ladder, the NaitXS, also lacks a balance control. You would need to go to a Supernait to get one. It’s a great amp and they are usually easy enough to find on the used market, if you have the budget.