Does Smile Plus flight to BKK as part of Thai Airways Business Class allow fast lane?
 

I plan to fly Thai (TG) for the first time soon on a short haul flight within Southeast Asia, and want to experience Business Class, but the flight to Bangkok will be operated by a Thai Smile (WE) which doesn't have Business Class but Smile Plus class.

Will I get the voucher/pass to go through immigration in the fast lane / priority lane? Are there any other things to consider?

I find conflicting information on Google from different past years, so I wanted to hear if anyone experienced this recently...

TG does not give out vouchers for fast track. Their Business Class passenger can use TG's own fast track, which is in Arrival hall East (opposite gate D4) on the left side (description and photos you find in according thread)
I am not sure whether Thai Smile Plus passengers can use this. I would encourage you to try and report back.

Thai Smile does not give fast track immigration either departing or arriving. I think it is clear on their website when booking, but to confirm my wife asked when she departed Bangkok on Thai Smile Plus last year and was told that there is no fast track immigration. 2nd and 3rd times same. As she is Thai makes little difference. If departing access to Thai Lounge is given and extra luggage can be a benefit (especially to my wife when travelling wthout me!).

Even if the ticket was bought on Thai Airways (TG) ?

Suggest you call TG or WE to ask. It may be unlikely that WE has fast track passes on board even if they should.

As an aside if buying a WE flight from TG it will be more expensive, but as you are doing multiple flights assume you have bought the best deal.

Just called. They have no clue.

if you booked on the THAI website and the flight has a TG flight number, then the answer is no. And if you booked on WE's website, the answer is still no.

from THAI's (TG) website there is no code-share business class with Thai Smile (WE), and since TG doesn't offer fast track to PE passengers, my assessment is that it won't for WE code-shared flights:

I would expect the answer to be no as well. There certainly are no WE related info on TG's fast track eligibility signs at the airport.
Out of curiosity, is it a point to point flight with BKK as the final destination with WE out and TG back, or? - Might be a silly question, but only asking as you said "within Southeast Asia".