I have tried viadrmums before, and yes, it works better and pretty stable compared to viadrm (I wasn't sure which was is more up to date though). Nevertheless, I believe it may be still useful to find the cause of this particular bug if it won't take too much effort.
On Mon, Jun 18, 2018 at 9:20 AM, <coypu%sdf.org@localhost
> The following reply was made to PR port-i386/53364; it has been noted by GNATS.
> From: coypu%sdf.org@localhost
> To: gnats-bugs%NetBSD.org@localhost
> Subject: Re: port-i386/53364: System crashes soon after X server is started
> with viadrm driver
> Date: Mon, 18 Jun 2018 06:18:25 +0000
> Please try the newer drm via driver.
> #viadrmums* at drm?