I came here to ask the same thing.

The Lightcarbon LCG071-D I have has enough space for 700 57mm/ 29 2.1" mtb tires.

Not according to their website.  Is your real world experience different? Thanks.

"- Max tire clearance 700*41C or 650B*1.9''"

I build up 3 Spycycle SP-M016 hardtail bikes for my 3 boys to ride NICA.  The geometry is spot on progressive w/ 75* and 67* at 450mm reach size large. Only problem for you is they don't make an XL.  Their latest hard tail is not as progressive but pretty decent at 74.5* 68* 475mm reach with UDH. I've been very satisfied with these 3 bikes. Threaded bottom brackets and so far so good. My 15yo rode his pretty darn hard and fast.
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This frame has Cannondale-style "kingpin" rear suspension. However, from what I understand, that suspension only works because because it's paired with thin flexy chainstays.

However, the G1055-D seems to have beefy chainstays, that don't seem flexy at all. What are your thoughts on this type of suspension? Is it going to work, or is it just a gimmick?

I do agree that chain stays are pretty beefy.  That said, this is just a render and we have no idea what the finished product will look like.  Not very many bikes have chain stays that are as beefy as those in this render.

Cannondale site has very little marketing claims on how the flex is created but several reviews I found online report that the flex is achieved more from the seat stays.  This makes sense as you probably would not want too much flex in the chain stays as this would rob you of power.

"According to Cannondale, the combination of the seat tube-mounted pivot and integrated flex zones in the seat stays, seat tube and rear section of the top tube provide active shock absorption at the saddle and rear axle." from granfondo mag.

If the Topstone had 50mm tire clearance I'd probably get one and try it out but 45mm is just not enough IMO.  I'd prefer 55mm but I guess that's not happening.

I just started another thread in the gravel section about the Yishun G1055-D Adventure Gravel frame. It has 50mm rear suspension that is just a pivot the Topstone but has clearance for 700x50mm.  This with a Rudy or Lauf is very intriguing to me.

I'm interested in some of the new gravel bikes coming out with rear suspension.  like the Yishun G1055 that is similar to the Topstone but with better tire clearance. This amount 100mm seems to be overkill though.  Especially with the limited tire clearance.

Also, some of the geometry just doesn't seem to line up.  55mm BB drop is less than some hard tail MTB's.  It just seems like if you are clearly going for more of a drop bar mountain bike then the tire clearance and some of the geo numbers should be more aligned.

Has anyone seen anything similar to this Voicevelo Major G? This frame looks amazing and has UDH. I just can't wrap my head around paying over $2000USD for a Chinese brand name frame. https://voicevelo.com/products/g-major?VariantsId=10041