TBH, I don't own any other Mediatek based routers, so can't comment on slow upload issue. I've never witnessed it on old Atheros based routers I own.
Unfortunately, I cannot remember for certain whether I tested 'pseudo bridge' mode (see earlier post) when I did briefly own a C50v4 last summer. I think I did test it but any test would not have been for more than 15 minutes to see how it compared to my Home Hub 5A operating in same mode connecting wirelessly to my main wifi network. But I could be wrong.
The open source wifi drivers used in OpenWrt will always be inferior to closed source wifi drivers included in OEM firmwares. I'm no expert but I don't think the Mediatek wifi drivers are as mature as ath9k and ath10k which have been around for far longer.
The C50 is a low cost router at end of the day. Have you considered spending a bit more money and buying an off the shelf mesh system such as Tenda MW3 if it is available in your country if coverage is more important than speed?
Otherwise, you may have to look in the 'Hardware Questions & Recommendations' sub-forum for further advice on hardware.