Tape the seams. PL300 bonds to the foam well, but isn't flexible enough to accommodate the shrinkage of the foam over time. In 25 years a 48.0" wide sheet of XPS will be between 47.5 - 47.75" wide which is a big improvement over the XPS manufactured in the 1970s, but sealing with adhesive just won't cut it. Use the PL300 to hang the foam on the wall, and snug the studs up firmly against the foam. (Some XPS is manufactured with ship-lapped edges, which won't have quite the temperature striping of well aged square edged XPS since there is always at least half-the thickness, not a gap.)
To hit R5 XPS is relying on the effects of the HFC blowing agents, most of which will escape the foam over time. The "lifetime warranty" is only for 90% of the labeled-R, or R4.5. But at full depletion it will reach R4.2, the same as EPS of similar density. It's greener & cheaper to just go with EPS, or better yet polyisocyanurate, both of which is blown with variations of pentane, which are far more environmentally benign. The chemistry of polyisocyanurate polymer has a somewhat lower overall environmental impact than polystryene, the polymer in EPS and XPS.
For the above grade section R4.5 is not quite sufficient for dew point control on R15 in the studwall in Cleveland's zone 5A climate, but if you go as air tight as possible on the wallboard the kraft facer will offer substantial protection against excessive wintertime moisture accumulation. The wintertime temperature of the below-grade section of each stud bay will be warmer, the fiberglass will wick and redistribute a modest moisture burden accumulating in the upper part of the wall to the warmer lower part, where it can dry toward the interior. It would be better with R6 though. The IRC prescriptive minimum for above grade R13 walls would be R5 in your climate zone, but you have R15. The simple math would be (R15/R13) x R5= R5.77.
Ideally you'd be able to insulate the sides of the window opening too, but it will impinge on the glass a bit, or at least the mortar line. About the best you can do thickness-wise is to glue 0.5" thick fire-rated foil faced polyiso (eg Dow Thermax), with 1/4" hardboard (eg Masonite) glued to the polyis0. In this instance you'll want to put a continuous bead of PL300 between the foam and foundation at the exterior edge side, and tape over the other seams with a high quality foil tape such as Nashua 324A (available at most box stores.) The hardboard has a very smooth finish that can be painted with standard interior latex. If using a non-fire rated polyiso it needs half-inch wallboard as an ignition barrier on the foam, which would be 1.0" thick stackup. A half inch of polyiso is only R3, but that's a gia-normous improvement over R0.