Here's a picture of open cubic player.

The Video RAM issue is strange. Those cards do not use linear framebuffer stuff that can depend on hardware design or bus type, so the OLB ET4000AX should look exactly like an ISA ET4000AX to the BIOS, just faster. Thus I don't understand why the RAM size detection fails with the newer BIOS. The Open Cubic Player clearly is a BIOS issue: OCP tries to switch into 50-line mode by loading an 8x8 font, but the BIOS you use does not resize the character cell to 8x8 after loading that font, so you stay in 80x25 mode with the top half of the characters being from an 8x8 font, and the bottom half the characters being the bottom half of 8x16 characters.

As you said, you had issues with the video RAM - maybe you still have video RAM issues and the new BIOS falls back to 256K because it detects that the full 1M does not work correctly.

For VESA support, I recommend you use the original TSENG VESA driver, which should be obtainable at various BBS mirror sites. Look for TLIVESA.COM (TLI = Tseng Labs Incorporated).

Like the Cirrus 542x chips, the ET4000AX only has a 16-bit CPU bus interface, so this will limit the throughput.

There are two variants of the Opti Local Bus: In one variant, the OLB slots are fully ISA compatible, but lack in local bus support, in the other variant, the 486 local bus is quite completely mirrored to the slot, but some DMA and IRQ channels are no longer available. Only the "full local bus/limit ISA" variant of OLB supports the "BS16" signal that signals to the processor that the request was handled by a 16-bit peripheral and if the request crossed a 16-bit boundary (a full 32-bit request does so), the 486 needs to repeat the "second half". You got the OLB ET4000 variant that splits 32-bit cycles on the card, and does not need BS16 support, so it should be universally compatible with all OLB implementations. I got a simpler one that requires BS16 support on the mainbaord, but at the same time, I got an OLB mainboard that only has the wrong slot variant...