I think the level at which you get those items is rather 5/10/15.
I don't remember exactly what's the supposed wealth per level (not in SRD), but at least I think for level 8th it was 27000gp, which is enough to buy a +4 item but it would cost 60% of your resources. That would be worth for a spellcaster, but less for the others.
In my experience, it is more common to get another item to boost a secondary score, but YMMV.
I think it would be more or less ok to give +1 every 3 levels. I would give even +1 every 2 levels if I was sure that players would use it to boost secondary scores, but that would rarely happen (unless they rolled the scores and want to get rid of odds)
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Why are you getting rid of the stat-enhancing items? Is it because you dislike the idea of an item becoming more important than the character? If that is the reason, I sympathise with it
One thing you may want to consider is to use more often magic Tomes and Manuals, which are more similar to permanently improve one ability. It would require some effort from you however because (1) their market price is totally off for your purpose and (2) the inherent bonus don't stack. You may want to just overrule the second.
If you are brave enough to go farther, you may rule that more than one character can benefit from the same book.