As you know, all the load/charging should be across the whole bank and not split among battery sets ("tapping") If some of the wires now are too short to do that, you can add more wire lug to lug or however. Buss can help there.
With the GC15s you do need to check SG after even one 50-90 since the SG does not get right up to baseline SG next time you go for a full recharge. Have to do an overcharge. Usually checking one battery up front you can get at will do though.
For the platform mentioned, one way is to use a garage style tool box carrier dolly on four caster/wheels built up to the correct height to be even with the battery tray.
With 6s you can arrange them so the series wire end posts are at the back, making the parallel link and load/charging posts all at the front for easier wiring. On a tray that can also make the posts that would take a portable charger up front where you can get at them.
