There are recipes that call for stepped mashes but afaik they are generally 'historic' and the grains we use today will all convert in within a reasonable range of 66-68C, however some may argue there are mouthfeel and body qualities that can only be achieved by stepping the mash, Im not gonna argue about something i dont know
I think some larger systems need herms/rims to maintain the mash temp where insulation and heat retention is impractical, especially with large metal heatsinks attached to feed and drain the system as well as cip hardware.
And If you want a herms kit build by all means go for it, however rather than throw large wedges of cash at a nice shiney set up, perhaps knock up a copper coil you can sit in a jug kettle to experiment with, i will admit to having a lil fiddle myself, fnar fnar!! and i used a copper coil of 10mm tube circa 2m in length coiled up to a 3-4" diameter which achieved 100% heat transfer from its 8 pint kettle element heated bath with a 10 c difference set between source tub and target temps with a 3l p/min flow..
When hooked up to a brew it was a bugger to set up even with a variable voltage pump psu, and after 30 mins or so of trying to balance flows i just gave up and have shelved it since..
Thermopots do hold good temps, but take a lot more preheating and perhaps a hike in strike temp too, Mine went from 71c in a plastic coldbox to 73C in the larger thermopot.. and also factor in upto £100 for the additional h/w FB, bottom drain fittings and valve.
just my take but i would have a brew with the kit you have, and possibly forget the 30l hlt, use the boil kettle to hit strike temp and volume, and sparge liquor, then drain into FV's you can then pour back into the kettle once the sparge has completed. It will add perhaps 20-40 minutes to the time elapsed between start and finish,and add to the cleaning up a bit, but will allow you to decide just what extra kit you need to improve the brewday to fit how you want to brew.
fyi i added a bit of clean dry copper back into my 100/80/100 SS build after noticing a lacking of something in the output.. I drop a bit of clean copper into the boil kettle..