However neither this cheaper retail version is listed at OpenWrt supported devices database nor I could find any information about it's hardware details at techinfodepot.
In fact, I found a bit of more information about it's cpu at guru3d saying:
The RT-AX1800U router uses MediaTek MTK7621A (dual core / 880 MHz) CPU, with 256 MB RAM and 128 MB flash memory.
So, the only difference, what I understand, Asus RT-AX53U uses MediaTek MTK7621AT and RT-AX1800U MediaTek MTK7621A cpu. And musashino's reference declares that he ate the pouding and that extra T ingredient at cpu name's end doesn't change the taste at all.
I will wait some more to be absolutely sure if this retail version is OpenWrt-able then reconsider the purchase. Thank you in advance for your further comments.
PS: Although seems solved, I wonder why Asus RT-AX1800U is not listed at supported devices database.