ESP-IDF v5 version of OVMSv3 is now officially booting and seems to work fine (then again, I'm not doing any real-world testing for lack of time / car at the moment - it's here that you can help me) with a few caveats detailed in the PR or the README.md.
The branch https://github.com/llange/Open-Vehicle-Monitoring-System-3/tree/experimental-esp-idf-v5 is now ready for testing, sharing, studying, ...
I created a draft PR https://github.com/openvehicles/Open-Vehicle-Monitoring-System-3/pull/810 (just like for ESP-IDFv4), in order to show the diff and gather feedback. Not to be merged as-is.
I'll try to break it into smaller low-impact / easy-to-review PR's that could be merged as-is in current master (warning corrections, header definitions, xxprintf type impact, etc...), so that the big pieces could be more easily reviewed.

Please note that those branches / PRs are still Work-In-Progress, and that I may certainly (and wrongly) `push --force` some changes as I rebase, update or split the big PR into smaller, easy to study PRs. I'll try not to but can't promise :-)

So if you checkout the branch, please be warned and know your git commands to either reset or rebase your working copy...

Next on my list are, in no particular order:

Have a few patches/bugs fixed in upstream wolfSSL (in progress) in order to enable
Check to see if we could upgrade mongoose (not absolutely necessary because it works / compile OK with a small patch)
Fix the remaining warnings

And change the ADC implementation that is deprecated now
deprecation of esp_vfs_fat_spiflash_mount / esp_vfs_fat_spiflash_unmount

Fix the WIP WIP WIP commits
Resume work on https://github.com/openvehicles/Open-Vehicle-Monitoring-System-3/issues/752

Boot time warnings to study and check if OK:

W (1091) esp_psram: Virtual address not enough for PSRAM, map as much as we can. 4MB is mapped
W (2132) ADC: legacy driver is deprecated, please migrate to `esp_adc/adc_oneshot.h` (<- this one is easy to understand...)

Additional instructions:

When first building, your sdkconfig will be converted as some config options did change betwen ESP-IDF v3.x and v5.x.

Run `menuconfig` and :

Select FREERTOS_ENABLE_BACKWARD_COMPATIBILITY(=y) "configENABLE_BACKWARD_COMPATIBILITY"
Unselect MG_ENABLE_SSL(=n) "Enable MONGOOSE SSL/TLS support"

If any of you with some free time could be nice enough to try to reproduce such a build and share the experience here. Let me know how the instructions could be updated and what issues you are facing.