All MNVOs I've checked in the U.S., including the one I use at the moment, all have had eSim support for a while. What you might be confusing eSim support with is the eSIM Quick Transfer. My transfer was instead done in the MNVO's own app and took less than a minute to generate a new eSim, invalidate the old Sim, and add the new eSim to my phone.
Edit:
For anyone who is curious:
Carriers with eSim Support in U.S.,
as listed by Apple:
- Appalachian Wireless
- AT&T
- Boost Mobile
- Carolina West Wireless
- Cellcom
- Chat Mobility
- Consumer Cellular
- Copper Valley Telecom
- Credo Mobile
- Cricket
- C Spire
- FirstNet
- GCI
- H2O Wireless
- Metro by T-Mobile
- Mint Mobile
- Nemont
- Nex-Tech Wireless
- NorthwestCell
- Optimum Mobile
- Pioneer Wireless
- PTCI
- PureTalk
- Red Pocket
- Republic
- Spectrum Mobile
- Straight Talk
- Strata
- Thumb Cellular
- T-Mobile USA
- Tracfone
- Truphone
- Ubigi
- Union Wireless
- United Wireless
- US Cellular
- Verizon Wireless
- Visible
- West Central Wireless
- Xfinity Mobile