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

Keep the Spur, invest in a rear shock with remote lockout and some lightweight wheels with Berd spokes. I have Berd spoke wheels and in my opinion they are fantastic.

63 degree head angle MUST be a mistake.  I don't think it's even possible to have a 73.5* seat angle and 63* head angle...

I proper XC bike should be in the 66-68* head angle and 75-76* seat angle or steeper like Scott Spark have 77* seat angle. Some people might prefer a little older school geometry and there are more Chiner frames around with 69* head angle and 74* seat angle.  That would not be my choice as I use an XC bike more like a 'down country'/trail bike if it has proper geo with good reach like an Epic Evo or Scott Spark.  If you don't care that much or only race then the older school geometry could work just fine.  But the bike on the OP post seems way off to me.

I built up a size large for my son to race cross country in middle school. So far so good.