First of all Sinovoip (BPI) works quite closely with MediaTek on the designs - in fact MediaTek sort of views BPI's boards as their demonstration platforms and gives them technical assistance. Since MediaTek pays an OpenWrt developer to work on WiFi drivers, this pays off for BPI. Also Dan Golle does a lot of work getting BPI stuff into OpenWrt.
BPI's boards get support in OpenWrt very quickly. The basic device tree support for the new R4's Filogic 880 went into OpenWrt two months ago.
People say that about the R3 too. And sure, if you look at it from one perspective, they are overkill. But on the other hand, an R3 costs only a third more than a Redmi AX6000, and if you were to walk in to Staples and try and get a WiFi 6 dual band that supports OpenWrt, you'd be lucky to get out of there and ONLY pay as much as a BPI-R3. You can do so much with them, to me it doesn't make sense to save $40 and have something that can only do one thing when for just a little more you get something you can throw at almost any requirement.
For the R4, SFP+ modules should actually be a little easier to find right off than SFP was for the R3 in the beginning. Getting 2.5Gbit SFP for the R3 was a little hard at first because they were already becoming a bit scarce in the wild before the R3 came out. There are lots of SFP+ options out there. @Boromini's right, the generics usually work.
There are a few on Ali Express - since the R3 came out, a few vendors have drafted off its popularity to revitalize 2.5GBit SFP sales. Just search for BPI-R3 there. Here are a couple:
- 2.5G-T Copper (works at 100M/1G/2.5G)
Select Generic and in the order notes tell them for a BPI-R3 - 2.5G-BX Optical