Do you have any advice to buy on Taobao, which agent to use ?

I would use Lunfeng, this is basically the same company. Lunfeng in China is 'Wheeltop' to the rest of the world, not sure if it's just a different name or the parent company.

Going via Lunfeng I got great help and assistance, they also linked me up with their Wechat support who have been great.

I'm fairly new to Taobao, but I've found their shipping rates to be super cheap.  I've just bought a powerful fan for indoor cycling which I had shipped via sea,  takes a while but cheap for large/heavy items and nice to have the option

I've been using waltly Ti frame with their supplied carbon forks for 2 years without issue. I did assume they purchased the forks from another local supplier... never thought of Speeder, but interestingly I've also been using Speeder wheels on the same bike without issue.

When I ordered my Waltly frame I asked if they had any 'off the shelf' gravel bike frames, they shared a 'standard' design and geometry.... the geometry was exactly the same as a model from a very well known Titanium brand.... I'm fairly confident that Waltly don't make frames for this brand, but I suspect the design and geometry was 'slightly influenced' by this brands model.

I am seeing two different style brake calipers being sold with GEX, does anyone have insight as to the difference and if it is worth the more expensive/newer style blue ones?

Sorry I don't know the answer, I've seen reviews with the older styles which have given good feedback on the braking, and yes, I also noticed the older calipers were much cheaper.  However, just to be safe I went with the new style so only have experience with the new model.  The new model is great, I can easily descend on bumpy trails using just one finger for braking.

This listing has some reviewer comments, one guy saying the upgrade is better braking, better sprint and wider gap..... if any of that makes sense or is to believed. https://detail.tmall.com/item.htm?id=834399534706&skuId=5584878208960

I'm also noticing that the price difference between the sets with the old/new brakes may have reduced a bit on Taobao

I'd be keen to try an 11-52t 13sp cassette.  Wheeltop did say that 52t will work on many bikes but not all..... it's just that they ultralight 11-52t cassettes are quite expensive

I had the same experience, via wechat they were super super responsive.

I've found this with a few Chinese suppliers, they use chat plaforms, eg. Wechat, whatsapp, skype, etc.  Especially WeChat, is more advanced and widely used for lots of things in China, much more developed and advanced that such platforms in Europe, US or Australia, for communications, payments, etc.

I think WeChat can do auto translation, but if you do want to translate, I've also found chatgpt is the best way to translate, chatgpt will understand the context and translate accordingly.

When you say 'Shimano compatible', I assume you mean HG (Hyperglide) type.  I use a Sunrace but there are loads of brands that do them, no idea why but 11-44 is not common, 11-42 and 11-46 are more common cassette types.... I think it's better durability to get a steel or mostly steel cassette, if buying from aliexpress I've been spedao recently, they seem to work well and lightweight..... however, I have not tried many other aliexprss brands, eg. ZTTO, Goldix, Sunshine, etc

I've gone a very similar route and my bike is almost exactly where I want it now. My idea was a dual-purpose bike, (i) off road gravel and fire trails, which around here in Australia can get quite rough and bumpy, (ii) medium pace group ride and touring road bike.  Note that my 'ohter bike' is an agressive lightweight climbing bike, so I wanted a contrast.

For the frame I went Waltly custom titanium, based the geometry/design on a Moots frame I liked.  I was going to go racy carbon but wanted Ti for touring strength and to scratch the itch of 'Ive never had titanium'.

Here's the remainder of the build:

• Lightweight aluminium bars and stem with Redshift Cruise Control Grips (from AliExpress).
• Wheeltop GeX electronic groupset with the 93mm long cage upto 51t cassette (loving it, allows me to run and swap between different speed cassettes)
• 1st weheelset: 650b Lun Grapid lightweight gravel wheels (ex demo).  I run these with 45-50mm tyres for rougher tracks, I went 650b because I'm only 167cm.... plus the 650x5omm has a similar rolling circumferance to 700x32. Cassette: 12 speed, 11-50t very wide cassette, ideal for offroad
• 2nd wheeset: 700c with 21mm internal road wheels, 40mm front and 50mm rear depth. I run these with 32mm or 38mm roac/slick tyres.  I love the 38mm slicks which can also happily be used for smooth/champagne gravel.  I got these from Speeder Cycling direct, very good price.  Cassette: 11speed 11-42t for closer gearing and enough spread for road riding
• Crankset: Xcadey 165mm power meter crankset with oval chainrings.  Crankset direct from Xcadey and oval chainrings from Aliexpress
• Last week added a dropper post and just bought suspension forks because they're were in sales at 66% discount, not sure these are needed

With the above I can swith between comfortable but fairly fast road and touring bike to plush and slightly monster gravel bike by swapping the wheels.  Note swapping the wheels changes the aero setup, tyre rolling resistance and weight setup, the gearing, etc.

If I were to buy the wheels again then I would go with an XD rear hub body giving the ability to run 10t cassettes, and I think you can get an XD to HG adapter but not the other way around.

Photos if interested or useful