one by one mate:
1. filters:
for high end UV/CPL try PHENIX, it has 2 series SP (Cheaper) and EX (more coating and more expensive)
for cheap but reliable try Tianya, it uses SCHOTT glass and has very good coating too, but the craftmanship of the alumiunm ring is less good.
2. Strap: depend on what do you want to strap on, SLR or Mirroless or P&S
3. Batter: try Pisen (en.pisen.com.cn), it's the OEM behind many Japanese brands, battery life usually longer than original ones.
4. L-bracket: again depend on which model you are looking for, SIRUI/Sunwayfoto makes medium priced ones for most DSLRs, Kangrinpoche is the top brand highly priced but world class making. There are many other smaller brands making L-brackets for Mirrorless, since they are all CNCed so precision is not a problem.
5. Flash: like you said Yongnuo is doing good.
6. Tripod: I wouldn't try cheap brands, best from China is Kangrinboqe, SIRUI and BENRO, I found there are some 3rd class brands from China very popular in Singapore like Selens, don't try those.
Hope the above helps.
Just a perspective from China since I am based here:
For filters, the strongest in China now are Nisi and Haida. Never heard of Phenix though. Tianya quality is not very good actually, they usually go for low end market.
For Straps, it is a mess free for all with bunch of no branders.
For Battery, Pisen is one of the major players but their batteries do not last longer than original batteries. Neither are they OEM for Japanese camera brands. Pisen is just a big battery manufacturer, supplying Li-on batteries for every purpose you can think of, from cameras to powerbanks etc... Even in China, most folks still buy original batteries, because they are not that much more expensive, but there are tons of fakes going around.
For L/QR plates, Sunwayfoto, Sirui and Benro are now the top. There is a new and upcoming aggressive brand called Marsace. They are not cheap but also offer some very hard to find long plates and stabilizers for birders. Low end market you will find iShoot. Kangrinpoche is no longer a major player after they were sold to another group (IIRC Shan Shen group) a few years ago. Build quality has suffered, and they are not very quick to release new plates for new camera models. Their sales have dwindled in the face of very fierce competition, also due to their lack of internet sales strategy and one price (high price) policy when selling in the domestic market. Even their stores in China, they seem more focused on selling ballheads from other brands, and their stores are usually dead quiet.
For Flash and other accessories (triggers, remotes, cables, etc): Yongnuo and Pixel are at the top. Yongnuo edges out Pixel in flash, while Pixel edges Yongnuo out in other accessories. A new and upcoming brand named Sidande is coming up very strongly now.
For tripods, King is shared by Benro and Sirui. Benro used to be the defacto king in China, with Sirui leading in oversea sales. Recently Sirui surged in the domestic market very strongly, so it is up for grabs who really is king now, both offer very good quality, with Sirui edging out Benro a little more, but Benro has more variety and selection. Other players are Jusino and Fotopro each with pretty good quality. And players in the lower end market includes Beike (QingZhuangShiDai), Weifeng and Yunteng. Like I said before Kangrinpoche is dead, and does not offer tripods in the first place. Their website has been unchanged since many years ago, have some new products but very limited, and often released late to market, their quality is no better than other offerings from new players, their prices are high. I doubt they will survive much longer.