Looking For Line Advice.

Biggest benefit is when stripping for a presentation over majority of each cast. Back close to and even into the rod tip and guides.. over entire outing. Common on smaller water (for me) Just a few feet of line plus leader outside the tip-top and head will feed right back out by utilizing 'line stick' on the surface. Once familiar this can be incorporated with your 'sweep' right into the set-up / D-loop of your next cast. With a smooth seamless (integrated) connection it's simpler and more pleasant (my opinion) than junction loops clacking in and out of the guides continually.

Couple observations: Have never wore a running line beyond useable shape prior needing to replace entire line.
When stripping and especially while assertively retrieving line to set-up next cast keep your rod as in-line as possible. Hard / severe right angles of rod to run line will accelerate much more wear. Even if a guy did toast the run line just below head (highest wear area) it's a simple matter to cut the head free and fix a loop in it's tail.. then move into running line of choice. Or just section out the damaged area of runner, re-loop and reuse. Point being the entire line is not shot, head is easily salvaged.

Have never noted a need for more distance (with factory integrated line) in places the Ambush is line choice. Easy-breezy out to 50' and very capable of more.

I've never over lined an Ambush. They seem pretty spot on for single hand rods by matching line class numbers.

Dug around a while with searches but was unable to come up with the vid clip I wanted. Couple years ago someone posted one of @Pat Lat spey casting with an Ambush. Recently was long term loaned a go-pro but have not even began attempting to figure it out and now it's full blown winter.

The 5wt Ambush is plenty capable. It should turn over Polys or Tips of up to 70gr (lighter likely better) Thing that you have going for intended fishing is use of fairly small non-weighted flies.. that helps big time.

I actually fish the 5wt on a pretty beefy 10' Scott 6wt, more like a 6.5, with 8' - 10' of Airflo's 10 gr per foot Customcut tip material and fairly large (unweighted) Sculpin patterns. The casting stroke used is faster, more of a 'touch-n-go' style than Skagit, but it works well for me.

Ambush has a color change at rear of head for visual reference. It cast pretty well overhead, no need to put much power to it.. just let the rod load deep on the back cast, bring it ahead smoothly and STOP fairly high.. she flies.

Back to over lining; you mention RIO's Single Hand Spey. I bought one in 5wt specifically for an old San Fran Scott G 10' 5wt. The line over head cast very nice. I didn't have much luck getting it to Spey cast though.. just felt 'light' to me and as a rule I prefer things on the light side. Would have loved to of had a 6wt on hand to try at same time. "Simon Says" match line numbers straight up for single hand Spey casting.. but it didn't work for me. My solitary experience (so far) with RIO Single Hand Spey line.

Scientific Anglers offered a couple Short Head Integrated lines for a while, not sure if they still manufacture them or not. "Adapt" was their premium offering but also "SBT" (Short Belly Taper) were available. I have an Adapt but it's still new in the box so no first hand intel.

The best news is: No shortage of good options these days.. modular or integrated.