Also, if you're still exploring options, you can check out the VGRO-75 drinking water filter. Having a reliable filtration system can bring peace of mind regarding your family's drinking water quality.
I hope you've found the right solution for your needs and have successfully made the switch to a water filter system!
Just wanna add that the systems you mentioned from Stiebel Eltron and the SimPure T1-400 seem like viable options. The SimPure T1-400, with its tankless design, can be a space-saver, but as you mentioned, filter availability might be a concern.