R4BE

Desktop replacement laptops and sub 1000usd msrp do not go well together. The cheapest laptops from major brands with tb3 that i can think of is xps 13 or ideapad 730s, both of which may cost over $800.
 
I was able to find 730s on ebay from us seller for just over $800 ($815 iirc) and refurbished previous gen xps 13 qhd for sub $800 on newegg. You might want to look into specifications, particularly how many pcie lanes are allocated to thunderbolt as that might affect egpu performance.
 
If I were in your position, I'd consider selling both lenovo laptop and asrock desktop and going for thin and light notebook with decent gpu (2060 or higher) that way you get the best of both worlds. 
 

Desktop replacement laptops and sub 1000usd msrp do not go well together. The cheapest laptops from major brands with tb3 that i can think of is xps 13 or ideapad 730s, both of which may cost over $800.
 
I was able to find 730s on ebay from us seller for just over $800 ($815 iirc) and refurbished previous gen xps 13 qhd for sub $800 on newegg. You might want to look into specifications, particularly how many pcie lanes are allocated to thunderbolt as that might affect egpu performance.
 
If I were in your position, I'd consider selling both lenovo laptop and asrock desktop and going for thin and light notebook with decent gpu (2060 or higher) that way you get the best of both worlds.