About to spend a lot of money tomorrow on this system (am I making a mistake?)

Brother! That's Freaky! Furry! Fabulous!
If it helps, the message I posted in another subgroup where I give the coordinates of a 15 kW multimode inverter with a Voc of 900V / 38-68V dc which also has a counterpart for the US grid (but I don't know if the "US" form is already in production).

as a preamble I would like to specify that I have no interest in the brands I quote.

Let me introduce myself:
Resident in Switzerland, archaically "analog" and pragmatic, but passionate about autonomies, I have 19kWp of PV installed on the roof, 22m2 of solar thermal plus a double bell type wood boiler with 3500lt of accumulation, solar dryer of 20m2... etc etc all in self-construction.
Currently I sell all the PV production, but given the exponential "politically and economically catastrophic" evolution of the energy market, I want to install a multimodal inverter (15kW) / multimodal = hybrid + island / in place of my grid inverters (7kW and 8kW) with a ~ 30kWh bank.
My Voc's are around 600 to 700V.
A self-built medium voltage battery does not seem relevant to me.
For questions of durability, reliability and simplicity I would have gone for NiFe, but the level of plating of the current manufacture is not equivalent to those of 100 years ago, without counting that certain manufacturers solder the plates = pollution Fe through the solderings!
I also considered super capacitors, besides their price even on the long term, they require "BMS" (it would take ~ 60V capacitors) to remove them, which is not found on the standard market.

Conclusions it will be LiFePo4.

Inverter :
After long research, there is currently only one inverter that meets my criteria The "Next3"(EU) and "NextUL" (US), put into production in late 2021. Technically it is a real breakthrough: 15kW three-phase, hybrid + island, 300A charger, Voc 900V and especially 48V battery.
It's handmade and apparently it's modular.
Made by Studer, which for me is like the Rolls of the island inverters!
...but, I haven't dared to ask its price yet...lol
(Although if you add up the different devices for the SMA variant for such a configuration "5 to 6 different devices..." the price should even be cheaper!)
At Studer, as far as technical requests are concerned, for the average person they are not..., to say the least, very reactive... if there are more requests they may be obliged to make some efforts!

For a question of redundancy and control finesse, I think to configure in 16~17S 2P.

And with AMY VAN from https://szluyuan.en.alibaba.com/ which although slightly more expensive seems to me to offer the best technical and logistical guarantees which in this market is priceless...
Stock shortage until March + Chinese New Year and after discussions, prices will certainly increase again.

A question arises:
- Isn't the Eve lf280k element "supposedly" 6000 cycles a marketing effect, thanks to the compression of the bank? Compared to EVE 304AH (LFP) "so-called old technology" 3000 cycles for almost the same price, which would you choose?

I would have gone for TAO (designed by a sailor, "hand-sewn" and above all very extensive functionalities) but there is the problem of data protocols between the next3 and the BMS...? Not to mention the price, which is almost half the price of the bank.
If we turn to Chinese: very used in your community in the island we find JK BMS but what about the protocols?
If a charitable soul having been confronted with this aspect, could answer these questions... the apprentice piston would be relieved!

NB: For information, I have a lot of trouble and "not so much the desire" with the internet, I even had difficulties to pass this message lol, so I can mix the interests raised by my message, please excuse me and correct me.

Have a radiant day."

Translated with www.DeepL.com/Translator
Systems work best if BMS communicates with inverters/chargers. However, some systems are built without that.
Look into what BMS of your preferred battery is compatible with.
This is very important. Protects your batteries.
OP has left the building FYI
I am now trying to stay away from threads like this.
I think there is a limit on how far you can go to accurately and safely advise a person with limited knowledge and experience on a forum!
IMHO a large complex projects require an expert installer on site to evaluate what is needed and to make sure it is done properly.