This 2001 archive lists the drivers suggested at Epox at that time including drivers for Win9x.
http://web.archive.org/web/20010621203954/htt … oad_ep-d3va.htm
Looks like you should be able to use any VIA 4-in-1 up to v 4.43 for the Apollo Pro 133A chipset and WIn98 according to a vogons post VIA 4-in-1 driver experiences
Phils has v 4.35: https://www.philscomputerlab.com/via-chipset-drivers.html
Vogons drivers has a few: http://www.vogonsdrivers.com/index.php?catid=32
The Raid driver is trickier because the driver version should match the Highpoint BIOS version...
you can try this one: https://www.driverguide.com/driver/detail.php?driverid=49733
That is all I know and am probably wrong....

The root cause turned out was resource congestion. I didn't mention this, but during installation, the computer had ACPI enabled, IRQ3 reserved for Wake-on-Modem and all PCI/AGP/ISA slots were occupied.

I have removed almost all devices except for the ISA soundcard and AGP video card, disabled ACPI, disabled WOM and attempted to install Windows 98 again. This time around I still get the mysterious 5.25" drive out of nowhere, but at least now the IDE controller does not encounter any resource congestions and Windows 98 is able to work it correctly and detect the CD drive.

Nevertheless, thank you so much for the links! They will certainly prove very useful along the way.

Interestingly enough, I have tried to use HPT370 driver for Abit VP6 on the Epox D3VA. It doesn't make any problems yet.