If you are after LiFePO4, either DIY cells, or look at the BYD bbox system. For LiNMC look at the LGResu, or Samsung ESS. There are others but I've not had feedback on them one way or the other. The LiNMC chemistry appears to be more susceptible to degradation at high temps.
If you are looking to set the system up yourself, I have now been involved with a few systems using the SMA inverters, and it is a very simple set-up.
If you are using an installer, it's best to let them install what they recommend. Keep in mind that not all installers will either care about what is best for you, or be on top of the latest technology that is available.
You will be able to use both your systems simultaneously, just feed them through an automatic transfer switch, program one system to switch off at a certain state of charge, then it will switch to the other one.
I am in a similar situation to you, and at the moment I'm tossing up between the BYD Bbox, and DIY LifePO4 cells. Advantage of the BBox is integrated BMS that has direct comms with the SMA. Greater supply chain meaning warranty should be more reliable, and cell delivery time is short. (Some suppliers of LiFePO4 cells order every few months, so if you need them in a hurry you might be out of luck).
If you do buy a LG / BYD, and a SunnyBoy/Sunny Island combo, all of the components in your system will be readily available, and there are thousands of such systems in use worldwide, so level of understanding and support is good. If you have a component failure, it is a very simple DIY replacement.
If you get an installer and they use a mixture of components, or any components that require programming or are not very common, you run the risk of delay in the event of failure, or simply that the installer you chose no longer works in the industry. When I got my first system 5 years ago I was speaking to 4 different installers, only 1 of them still works in the industry, and they use different componentry to what they used then, and don't carry stock of what they quoted me on....
Good luck, and keep us posted on your progress.