I would recommend getting a Z270 board over a H270, especially one with two actual x16 slots (That run in x8/x8 mode with using both slots). H270 motherboards cost almost the same as a Z270 motherboard on the used market. If you get the right Z270 board and configuration, you get 10-gigabit USB and the ability to run 2 PCIe 3.0 NVME M.2s at full speed technically (via through an x16 slot and an on-board M.2 slot). Al KabyLake boards that feature 2 on-board M.2 slots cannot run 2 NVMe SSDs at full speed, only 1 can run at full speed due to DMI 3.0 (chipset) limitations. Sadly, with all dual CPU-linked x16 slot Kabylake boards, you cannot do bifurcation (Meaning turn the first slot into x4/x4/x4/x4 or top slot x8, bottom slot x4/x4), well in normal terms, you can only use 1 NVMe drive per CPU-linked x16 slot, even though technically both slots can run 4 total NVMe SSDs at full speed just fine. There is add-on cards that have on-board bifurcation but there either can run 2 NVMe drives at half speed for cheap or run 2 NVMe drives at full speed for an expensive price (usually costing double what the motherboard costs). If you use both on-board M.2 slots, both NVMe drives will usually automatically be set to half-speed even if your not using them at the time.
I mean sure Thunderbolt 3 is cool but the add-on cards can cost the same as the board itself on the used market. Most board makers expect you to install that card into the PCIe 3.0 x4 slot that's connected to the chipset, which is also shared with the on-board M.2 slot, so if your transferring between them, Its only 16-gigabits of useable speed. If you install the NVMe M.2 into the one of the CPU-linked PCIe x16 slots and you install the add-on card into the other x16 slot (Thus eliminating the DMI 3.0 problem), 32-gigabit of useable transfer speed is possible. Thunderbolt 3 can do 40-gigabit but those add-on cards have a PCIe 3.0 x4 interface, which limits the card to 32-gigabit max. Considering modern boards are starting to come with built-in USB4 ports and even Thunderbolt 4 ports, This old add-on card hunt is kind of gimmicky at this point.
TLDR: H270 boards are close in price with Z270 boards. With the right Z270 board, you get dual CPU-linked x16 slots (running in x16/x0 mode or x8/x8 mode) and even 10 gigabit USB. Kabylake boards (that even feature 2 M.2 slots) can only 1 run NVMe drive at full speed due to chipset limitations. Finding an Thunderbolt 3 addon card (If your board has a connector) is waste of money due to them costing the same as the board itself. Buying used is your best bet.