Too few IOMMU groups - split?

I bought AMD RyZEN 5 2400G, Gigabyte B450M D3SH and 16GB RAM. Some elder GPU AMD Radeon HD 6XXX I took for passing through. But I was unpleasantly surprised with the IOMMU grouping. I put the GPU in either first (16/16) or third (16/8) PCIe slot, and found that it was in both cases included in the iommu group zero. But other members of this iommu group cannot be passed though to VM neither excluded from host system. There were just four iommu groups (0 - 3), while in the tutorials and my other PCs I see twelve or more groups, and some PCIe slots are quite isolated in a single group. Questions: Does anybody knows: - If I should buy some other hardware? - If there are some kernel options to get more iommu groups (split)? - If do I have to use legacy BIOS instead of uEFI? SVM enabled, IOMMU enabled. OS: Debian Linux testitng, kernel 5.10, kernel cmd options iommu=on amd_iommu=on iommu group zero: 01:00.0 USB [0c03]: [AMD] 400 Series Chipset USB 3.1 XHCI Controller [1022:43d5] (rev 01) 01:00.1 SATA : [AMD] 400 Series Chipset SATA Controller [1022:43c8] (rev 01) 01:00.2 PCIbridge : [AMD] 400 Series Chipset PCIe Bridge [1022:43c6] (rev 01) 02:00.0 PCIbridge : [AMD] 400 Series Chipset PCIe Port [1022:43c7] (rev 01) 04:00.0 Ethernet : Realtek Co. Ltd. RTL8111/8168/8411 PCIe Gigabit Eth [10ec:8168] (rev 0c) 05:00.0 VGA : [AMD/ATI] Turks PRO [Radeon HD 6570/7570/8550] [1002:6759] 05:00.1 Audio dev : [AMD/ATI] Turks HDMI Audio [Radeon HD 6500/6600/6700M Series] [1002:aa90] 06:00.0 NVMe : Intel NVMe Optane Memory [8086:2522] group 1 ... 08:00.0 VGA : [AMD/ATI] Raven Ridge [Radeon Vega (Mobile) Series] [1002:15dd] (rev c6) 08:00.1 Audio dev : [AMD/ATI] Raven/Raven2/Fenghuang HDMI/DP Audio Controller [1002:15de] ... As you can see, the APU (GPU integrated on the CPU chip) is a member of the group 1.