This weapon (as written) may get less use than you expect at the levels you describe
You said that your players were "almost level 5," and that you hoped they would use this weapon "up to level 12-15." I understand that to mean you're hoping they will use this weapon at levels 5-11. Unfortunately, I think you might be disappointed by the results.
You likely are comparing this weapon to a standard Heavy Crossbow, which makes sense given your party's current abilities and armaments. In that respect, this weapon is a good upgrade. They both have the same average damage for a standard shot, but the magical weapon will have improved accuracy, and give various AOE options to a few attacks per day. At level 4, this weapon is likely appealing.
However, when the player reaches level 5 and above, the math may change. At level 5, a Ranger gets the Extra Attack feature, meaning they will be able to attack twice whenever they take the Attack action. However, crossbows have the "Loading" property which states (PHB, p. 147):
Because of the time required to load this weapon, you can fire only one piece of ammunition from it when you use an action, bonus action, or reaction to fire it, regardless of the number of attacks you can normally make.
As such, when this player reaches 5th level, they will be comparing the ability to make two attacks with a longbow with this magical crossbow:
- Aurel‘s Adaptive Crossbow: standard attack (1d8+5), average damage 9.4
- standard Longbow: Standard attack 2*(1d8+4), average damage 17
The bonus action abilities you have are very helpful, and likely will get used once or twice a day with relish, but this crossbow will likely not see regular use throughout the day.
At level 8, your player will need to chose between a final ASI increase to Dexterity, or a Feat. It's quite possible they will choose the Crossbow Expert feat, an excellent option for many ranged characters. If they choose Crossbow Expert, then this magical crossbow will lose its loading property, which will give it a leg up.
However, if the Ranger makes this choice, suddenly the magical crossbow will be competing with the simple Hand Crossbow. The comparison then becomes as follows:
- Aurel‘s Adaptive Crossbow: standard attack 2*(1d8+5), average damage 19
- standard Hand Crossbow: Standard attack 2*(1d6+4)+ bonus action attack (1d6+4), average damage 22.5
The slightly higher accuracy from Aurel's Adaptive Crossbow might make up for this difference, but at best you're likely breaking even with a Hand Crossbow's utility. And if instead of Crossbow Expert, the Ranger chose a Dexterity increase or the Sharpshooter Feat, the magic crossbow will likely get left behind.
Level 11 creates a particularly tricky situation if the Ranger chose the Hunter subclass. At that point, they will be able to use their Volley ability to deal AOE damage at will, so long as they have enough arrows. This will mean that most of Aurel's Adaptive Crossbow's special attacks will become a lot less appealing.
My suggestion: add some (more) versatility
Depending on the kinds of Feats or class features your Ranger has, different weapons are likely to be more or less appealing to them at different levels. If you want this magical weapon to see continued use, I think there could be a viable way to make that happen. The first step is to rename the item, and add the following (italicized) sentences at the start of its description:
*This magical bowstring can be fitted to any ranged weapon with the ammunition property that requires arrows or bolts as part of a short rest (which can be the rest used to
attune to this bowstring). At the end of that rest, and until the
bowstring is transferred to another weapon, the weapon becomes
magical, with the following additional properties... *
With this simple change, your proposed weapon will fit to whatever build this particular character has, as long as they remain fond of ranged weapons. It also fits with your (interesting) idea of having the weapon suit various different situations well.
Questions of Balance
Overall, with the above proposed changes, this weapon would be balanced relatively well! A Ranger already has various AOE abilities (like the Hail of Thorns spell), but is limited in their use by spell slots. This weapon will effectively give the ranger a few extra "spell slots" worth of ranged harm options per day.
Something that will be appealing about this weapon at every level is that its abilities can stack with whatever options the Ranger already is using, unlike most of the Ranger's inherent abilities (for example, the Ranger can't use Hunters Mark and Hail of Thorns at the same time, but could use Hail of Thorns and a Fire ability from this weapon, provided they used a Bonus Action at the end of one turn to activate one, and the start of the next turn to activate the other). This is likely to be appealing at all levels of use, provided the standard attack stays competitive with other weapons.
The one problem I can see is that this weapon will feel more appealing early on rather than later. The abilities it gives access to are very useful in and of themselves, but may seem less impressive when compared with spells the Ranger can cast at higher levels, particularly since most of the abilities of the crossbow can only be used once or twice a day. To help combat this, I have one final suggestion.
I'd suggest that as part of a short rest, you allow the Ranger to spend Spell Slots of level 1 to 4, and add a number of charges to the crossbow equal to the level of the spell for each spell slot used. This will allow them to get more use out of this weapon at higher levels, and will keep the game's economy more or less intact (e.g. a second level spell slot could be used to cast a 2d10 Hail of Thorns, or a 2d8 Fire arrow attack).
With these changes, I think this weapon will remain in use for many levels, and remain relatively well balanced throughout.