Water softener + riser rebuild questions

[citizen]

Hello, been lurking for a bit and absorbing as much as I could from the community. Will be embarking on a softener project that has ballooned into a master shutoff / riser / PRV / leak monitor project.

Looking into:

1. Water softener system (Clack/Fleck valve system from Quality Water for Less, AFW Filters or US Water Systems' Matrixx)
   1. Primarily to prolong life of faucets, appliances, reduce scale buildup
2. PRV (Wilkins 70XLDU or NR3XL, any others to look for?)
   1. House does not have an existing PRV and pressure seems high
3. Leak monitor (Moen Flo)
   1. Might as well add for insurance purposes if we are rebuilding the riser?
4. Replace shutoff valve (SS ball gate valves, recs?)
   1. Unknown age of existing one
   2. Mixed feedback of whether I should bother replacing it right now

Questions:

1. For a 4000 sq ft., 3.5 bathroom house, are we fine stepping down the 1.25" riser line-in to 1"? Consensus makes it seem like it'll be fine, and we are going to install a PRV that'll be reducing pressure anyway. Five adults, two kids. Or should we just pay up and stick to 1.25" for everything?
2. Reading about potential issues with mold, etc., I'm not inclined to install a whole-house filter; especially if I already have an RO system in my kitchen. Would anyone still opt for one?
3. Would you install the leak monitor after the softener right before water enters the house? So it would look something like:
   1. Line > split to sprinkler loop > shutoff valve > PRV > softener > leak monitor > house

Context:

• Water tested at 20–22 gpg (city claims 16–20 gpg)
• 85 PSI, surprised the house didn't have an existing PRV
• Already have an RO system under kitchen sink
• Prior owner at some point had an automated system (evidenced by an existing perfectly function drain line to sewer nearby) but then switched to a tank swap option; whatever she was using didn't seem to work well as the faucets and lines have a ton of buildup
• Already installed electrical nearby

Photos:
Sampling of some of the buildup even though prior owner had a water treatment system in place:

Prior owner's tank swap system:

Existing 1.25" riser, sprinkler loop, shutoff, double loop and house entry: