It will be fine charging the battery with cables connected. I charge my Nissan Leaf with cables connected often. The DC-DC has (a) diode(s) in the output which will protect it from harm. All you're doing is raising the battery voltage from circa 12 V to circa 14 V anyway; there is voltage present at the DC-DC output all the time.
Disconnecting 12 V, although occasionally necessary to reset various ECUs, is not a great idea for long periods of time, IMHO.
I have a smart CTEK 12 volt battery charger, that I have used to “Top Up” the battery on our ZS EV a couple of times.
It comes with an accessory fly lead that has two eyelet terminals, that can be permanently attached to the 12 volt.
The red positive terminal is attached directly to the + positive terminal of the battery, while the black eyelet - negative terminal attaches directly to a ground ( earth ) point on the chassis of the car.
At the other end of this short fly lead, there is a weather proof connector, that mates up with the leads on the CTEK smart charger.
Just very convenient, safe and quick to connect and use.
Much better and safer than using them crocodile clamps that tend to jump of the terminals !.
The charger does come with the crocodile clicks if you need them.
As said above, I have used this charger about 3 - 4 times now and not disconnected the battery terminals from the battery.
If I was using a high capacity garage charging unit, then I would consider disconnecting the - negative terminal first.
But a lot of the small smart / conditioning chargers are only providing a very low trickle charge over a longer period of time.
If you had a completely flat 12 volt battery and called out the rescue services, my guess would be that they would attach a commercial high powered jump pack directly to the terminals of the 12 volt battery, without disconnecting them of course.
So, using a low powered trickle charger with the battery terminals still connected is not a problem really.
I would purchase a trickle charger if I was you !.
