If it were me I would definitely do what you're talking about here, especially considering that two of those modules in series are pretty much the perfect voltage, but of course there's plenty of precautions one would have to take.
Primarily I would start by removing all the bridges on the pack you're working on. Beginning with the middlemost bridge, in your picture looks to be the battery right above where the clamps are, so 2nd row 5th column starting from bottom left. Then work my way out. Of course all of this using decent gloves, boots and just being overly aware of everything. This is not the time to be working in an uncomfortable position or having anything that could cause involuntary movement, be it a rolling chair, working on a cluttered bench, having anything on the floor you might step on or kick by accident. Any tool you use, you put away before getting another. if you don't have HV tools you can just wrap electrical tape around the shaft of screwdrivers to the tip, if you need to use a wrench (which I don't think you will), do that too, only one tip should be exposed.
Once you remove the middle bridge the pack becomes relatively safer since you cut the voltages by half, but regardless, you do not want to be making modifications to them while you have 240v on tap. Just remove them all doesn't matter if its tedious or whatever. If shit hits the fan I would argue its easier to put out a battery fire from a bridge that slipped and shorted, than to somehow overpower the 240v going through your neurons and into your muscles.
Probably what I would do is put tape (2" painters tape works fine) over all the exposed contacts and slowly peel it off, remove the bridge, then put the tape back on, once you finish all batteries will be disconnected and will all be protected from any accidental tool drops.
And after pretty much "deactivating" the batteries, I would just look into the wiring and see what I would have to do in order to series connect them. You will only have to re-wire the "master" battery (the one that will ultimately go to your inverter, and they can only be connected in series pairs, so only one battery plugged into the master, and none into that one. Any paralleling would have to be done externally.
Not sure how the wiring is from the pictures, so you'll have to double triple and quadruple check, but if internally, the input of the master is directly paralleled with the daisy chained batteries, you'd basically just disconnect both + and - that come from that connector onto the breaker, disconnect either + or - that come from the master batteries themselves, and connect them in series, if you disconnect the battery negative for example, you'd connect it to the positive from the connector, and then use the negative from the connector as your new battery negative.
Just REALLY make sure to check everything with a multimeter before putting the bridges back up. If you're sure its good, you have enough wire to move the + terminal over and basically use only one 12v battery, for testing. Do that for both master and slave and then see if you get your 24v at the master output cable. If so, you're pretty much ready to cut and wire the master to the inverters, and then bridge them up, close them, turn breakers on then switch the battery d/c on your inverter.
I have no idea if those breakers are designed for such high voltages, but regardless, your inverter has I believe 25a battery fuses, which are going to blow WAY before those breakers trip, considering the packs are rated for 60A continuous.
People can hate all they want on SLA but they're free and you have a pretty much best-case scenario here, why not? I did the same with our golf cart. Company had 24x 12v 24ah sla's that they bought for a UPS but the UPS was replaced literally a week later. Got the batteries, made 6 4s packs for 48v, put them all in parallel. Now of course, did they last as long as regular golf cart batteries? Honestly not really. Did they get the golf cart around, and last around 4 years before losing most of their capacity? Yes. But it was a lot of cycles during those 4 years. Not that 6v batteries would've lasted much longer either... Eventually family friend gave up on an EV conversion he wanted to do, and gave us 28 nissan leaf cells, which we used to make a 52v battery pack and OFC its 3-fold better than the SLA batteries, but at the time it's what was available. My point is, ESPECIALLY with new EV's coming out each year, it'll be maybe 5 years until we see second hand reclaimed car battery packs drop drastically in price.
Right here you have over 10kwh of SLA. Even if you "derate" it by half its still enough to run several fridges/freezers for probably over 24h, way more than enough to last you the night till the sun comes up, if you were to lose the grid.