Changing cranks from Praxis to Ultegra (BB advice)
If it is an M30 Thru BSA, then the cups will say 'M30 THRU'. As they say M30 and only M30 then they are the M30 BSA version. The latter suits crank spindles which are 30mm drive side, 28mm non-drive side, as the Zayante crank is. 

If they say anything else (like press fit) then (future readers) can match that to other models on the Praxis website before proceeding. For example, it's possible to get thread-together units for pressfit frames which don't look that different but have different wording on and are not BSA.

If BSA is confirmed, then the frame is threaded to the BSA standard. As the Shimano spindle is a smaller diameter (24mm) then yes you would need to change the bottom bracket, but this is trivial with a BSA threaded frame. That is to say, yes you unscrew the Praxis and screw in a Shimano. 

It does look like the Praxis BB may require a different tool to the Hollowtech II items, but the shop can check this and take care of it, maybe make them aware beforehand to avoid delay if they need to get a tool in. I think having the shop do the whole job is a fair idea, it represents minimal risk to your wallet, even if parts are more expensive at retail.

Happily you can mix and match the Shimano BB, new chainset and 105 LH crank.

I know of no reason to distrust Shimano BBs; the more expensive ones are lighter and smaller items which might make them look less 'rugged' perhaps, but they are tailored to perform a certain way, i.e. light and racy. 



If the bottom bracket cups say anything additional to "M30" then it is a different model.  As Weiwen Ng has sleuthed in a comment above, it seems 2018 Roubaixses may have either BSA threaded, or pressfit bottom brackets, so measure twice, order once if reading this in the future.