If I wanted to build a 2.5” micro which runs a 450mAh 3S battery, what components would be recommended? I want to build it around the RunCam Split Nano - I had a Larva X which I really liked but I would ideally like even a little more punch.

for a split-weight, i would choose at least 04 stator, 1105 - 1404 would be my choice, i would look for xing motors, they seems to be very nice. dave_c designed the „sanchez“ frame, his new practice focused frame is named „jenga“, there are some buildlogs on rotorbuilds.com, dave_c have nice examples. kababs framedesigns are aswell nice... a lot of frames on the market, i wouldnt pass 15g weight for the frame. i guess there is a new 26x26mm layout aio-board, there was a thread i guess from snow in the past weeks about. i dont own any working 26mm electronic, i personally like 16x16mm or 20x20mm but the most frames in this class are designed for 26mm electronics.
if i would build somethin at the moment, i would build an 1404motors with the new and rare 4inch props with t-mount, should be a power settup. but as you ask for 450mah 3s lipos, im not sure about, probably they are a bit to small for 4inch power.

The only thing I have flown in this class in the Larva X, on 2.5” props and 3S (I did try it on 2S but I preferred it on 3S). I am also aware of BetaFPV’s similar offering (except I think it’s 3”?). That is probably as big as I’d like to go.

The point of this build is to be able to fly it in places where a 5” quad would be socially unacceptable.

Basically I would like a board which fits on a frame and also isn’t AIO (as in, it allows an external RX and VTX) since I would like to use crossfire and the Unify Nano for 400mw video.

I’ll take a look at these builds - sounds like there are some ideas there to be borrowed

there are aio options without builtin receiver or vtx, only esc and fc in combination.
i would suggest 3“ or 4“, i guess both are able to fly in „5“ socially unacceptable“ areas.

https://www.airbladeuav.com/products/dia...tack-16x16
something i would choose. oscar have a lot of reviews of micro parts, i guess one of mamba 16x16mm stack aswell

edit: as you described „more punch“, you mean „torque“ „throttleauthority“ or more top end?
if you didnt try other props, lower pitch or more skinny blade should give more torque, authority for theottle. high pitch will give more top end, not sure what props the larva has; hq2535?
as the motors are pretty high kv, i guess lower pitch should give better results, twoblades ahould give better throttle authority less cost of topend speed, than a lower pitched triblade. hq65mm on the larve would be my test to get more throttle authority.

For the 2.5 inch build, here are my suggestions. This is a non-toothpick class build. If you want to go ultralight, then check out KababFPV's channel.
Emax 1106 motors
Mamba F4 micro stack
Catalyst Machineworks Massive Droner 2,5 inch frame
Emax Avan 2.5 innch props

Another option would be to go the 3 inch ultralight route. FPVcycle.com has the new TP3 build, which has the ability for HD micro cameras. In fact, the 3 inch ultralight may be a bit cheaper than a 2.5 incher because of the use of AIO whoop boards, cheaper motors, and cheaper frame.

One of the most important things for micro builds is to have a super reliable FPV setup. I have learned this the hard way a few times too many! A high quality video transmitter is the most essential thing here, along with a reliable camera.