lets do some math...
lets take the basic array 15, 14, 13, 12, 10, 8
you get +2 to 1 and +1 to another... I will end up with
16 15 14 12 10 8 (not bad I can totally make a character out of that)
we will give the 2 prof saves to the 12 and the 16
16 (+5) 15 (+2) 14 (+2) 12 (+3) and 8 (-1)
DMG
Use the Table. You can start with the monster's expected challenge rating and use the Monster Statistics by Challenge Rating table to determine an appropriate save DC for any effect that requires a target to make a saving throw.
Calculate the DCs. Alternatively, you can calculate a monster's save DCs as follows: 8 + the monster's proficiency bonus + the monster's relevant ability modifier. You choose the ability that best applies. For example, if the effect is a poison, the relevant ability is probably the monster's Constitution. If the effect is similar to that of a spell, the relevant ability might be the monster's Intelligence, Wisdom, or Charisma.
Don't worry if the save DCs aren't matching up with the expected challenge rating for the monster. Other factors can affect a monster's challenge rating, as shown in later steps, and you can always adjust the sa - DCs later on.
here is a redit post with a break down of monsters
Okay so at CR 1/4-4 (what I would expect to come across most often in the +2 prof days) you have 13 DCs
At CR 16-21 (what I expect most 13-17th level characters with a +5 prof to see) you have a 18-21 DC range...
so at the start you make your best save in an 8 your worst on a 14
level ups are hard to calculate... do you up stats or take feats (or multi class like I do alot to keep spellcasting from getting out of hand) but the prof is for sure... I think assumeing a +1 magic is not too far out there...
no stat increase
16 (+9) 15 (+3) 14 (+3) 12 (+7) and 8 (-)
up your prime stat to 20, and secondary to 18
20 (+11) 18 (+5) 14 (+3) 12 (+7) and 8 (-)
up prime to 18 and secondary to 16 and the 8 to a 10
18 (+10) 16 (+4) 14 (+3) 12 (+7) and 10 (+1)
3 arrays and we can do out what you need to make best and worst... so
DC 18 if no stat boost you need a 9 if some but not max you need an 8 and if you max it you need a 7.
Your best save on the low end of the spectrum is running on a treadmill
DC 21 if no stat boost you need a 12 if some but not max you need an 11 and if you max it you need a 10.
Your best save on the high end is WORSE for you then you started.
Worst save is either a straight roll or +1
DC 18 if no stat boost you need a 17 if some but not max you need an 16
Your save save is so bad you might as well not try... it WISHES it was ONLY on a treadmill
DC 21 if no stat boost you need a - opps nat 20 doesn't auto pass in 5e no save for you... if some but not max you need a nat 20
Your worse save might just be an auto fail, but if it's not it might as well be...
you go from (best save in an 8 your worst on a 14) 35%- 60% to (best needing 7-9 and worse 20) 60%-5%
you don't get better at avoiding threats that are level appropriate you get WORSE>