Reddit mentions of 1.25G SFP-T, 1000BASE-T Copper SFP, SFP to RJ45 SFP, Compatible with Ubiquiti UF-RJ45-1G, Mikrotik, D-Link, Supermicro, Netgear and More.

Only the 48 and XG have 10G SFP (SFP+) ports. You could link both ports and aggregate them, but I'm not sure it's worth the extra cost and effort.

That being said, the 10GTek ones on Amazon have worked fine for me.

Edit: Added link.

https://www.amazon.com/gp/product/B01LW2P72V/ref=oh_aui_detailpage_o00_s00?ie=UTF8&psc=1

i use these without issue.

Personally, I would just get an RJ45 module, like this one.

https://www.ebay.com/itm/Quanta-LB6M-10GbE-24-Port-SFP-4x-1GbE-L2-L3-Switch/263779314790?epid=17021561857&hash=item3d6a78ec66:g:TBwAAOSwUPZbMUHN

https://www.reddit.com/r/homelab/comments/7krr90/upgrade_your_quanta_lb6m_flash_to_a_full_featured/
https://www.servethehome.com/turbocharge-the-quanta-lb6m-with-brocade-turboiron/

Depending on distance you could use 10gig copper twinax

Copper switches are sometimes just a bit more but are more difficult to find. 10G copper SFP modules can sometimes be found for 50 to 75 on ebay and new for 100 - 150.

The switch, IIRC, has a few 1G copper ports but you can get a 1G copper SFP for 20 or so https://www.amazon.com/10Gtek-Ubiquiti-UF-RJ45-1G-Transceiver-1000Base-T/dp/B01LW2P72V/ref=pd_lpo_vtph_147_bs_t_1?_encoding=UTF8&psc=1&refRID=AWWXZ1YRZ967878Q6JPX

Official version: https://store.ui.com/collections/accessories/products/uf-rj45-1g-1

Third party: https://www.amazon.com/dp/B01LW2P72V/ref=cm_sw_r_cp_api_i_L--7CbKWG7945

Something like these?

https://i.imgur.com/LZCmXcS.png

https://www.amazon.com/10Gtek-Ubiquiti-UF-RJ45-1G-Transceiver-1000Base-T/dp/B01LW2P72V/ref=sr_1_3?keywords=sfp+ubiquiti&qid=1556912526&s=electronics&sr=1-3

if i use that on the XG could i just plug the other end of the RJ45 into a RJ45 port on the US-8-150w instead of paying another $21 for a 2nd transceiver?

I use the SFP and SFP+ ports on my UniF switches as regular switch ports...works fine no issues.

Here is the SFP to copper adapter I use: Amazon Link

Here is the SFP+ to copper adapter I use: Amazon Link

Just plug and play.

For the ERX-SFP if I wanted to connect the RJ45 from the modem to the SFP port for the WAN connection I would have to use something like this, correct? Would you recommend that over the Mikrotik hEX for routing over a gigabit connection?

That's what I ordered - the "for Ubiquiti" version of this.

I use one of these on my 16 port switch for UL to keep all 16 ports available for network devices. No issues.

10Gtek for Ubiquiti SFP Transceiver, 1.25 Gigabit RJ45 Copper SFP Module, 1000Base-T
https://www.amazon.com/dp/B01LW2P72V/ref=cm_sw_r_cp_apa_0yHJAbTJ2R4HX