Switching high voltage battery from series configuration to parallel configuration for charging
This is the universal conundrum of parallel vs. serial connection of cells/batteries, for both discharge, &amp; re-charge.  There are pros &amp; cons for each scenario - efficiency/losses (P=I^2.R), safety (Lithium is an unforgiving mistress), convenience / accessibility, longevity, equalisation.

RC hobbyists trade convenience with a willingness to disconnect packs from their gear, and then connect them to a charger, which gives them flexibility to arrange cells in whatever series or parallel arrangement makes best sense for each discharge &amp; recharge phase of their play-flow (work-flow - geddit? ;).

As you say, your application doesn't facilitate this physical flexibility - the cells aren't easily accessible, nor is changing harnesses.  You could engineer a complicated switching scheme with either relays or mosfets, to allow you to reconfigure the array for discharge &amp; recharge, but that's a lot of work, a lot of cost, and extra copper/mosfet losses.

If it were me, I'd suck it up &amp; go for a Li-Po recharge solution that supports the number of series strings with cell/pack-balancing capability (which of course is critical in series configurations).  That may mean you need to take your power source &amp; use a switching boost voltage regulator to get the voltage you need (i.e. more than the number of series cells * 4.2V) to achieve this, and then a charger to match.

If you really want to stick with hobbyist-level chargers like the one in your photo that can handle a handful of cells in series, then one approach I can think of is to power each of those rechargers via isolated DC-to-DC converters (in turn powered from your solar/MPPT supply, or bulk mains-powered PSU) - that lets each charger handle a handful of cells in series, and provides the isolation needed between each charger as the string voltage escalates along the string.  Here's some examples of the kind of DCDC modules I mean.  One challenge of this approach is how to control each of those rechargers electronically (instead of humanely) whilst maintaining the isolation needed between them, but that's a separate question :).