If I understand the issue, there is no fix the way you want one.
You can't wait to combine multiple purchases - the timer on the first sale starts with the first sale, and if you hold off beyond your handling time to ship that first item you get a ding. (eBay reps MAY have stated that you can call eBay CS and get those dings removed, but IIRC, that doesn't work out in practice)
A partial fix is extend your handling time to cover the timeframe you set for combined purchases (ie: 3 or 4 day handling if you have "Buyers can send one combined payment for all items purchased from me within 3 days" which is the minimum), but the eliminated the possibility of having TRS+ listings.
The way eBay set this up you can't really provide that sort of customer service and retain TRS without a long handling time, which makes your delivery windows look long to buyers (although if you have a longer handling time like 3 days, but ship in one day most of the time, eBay will adjust the delivery timeframes shown to buyers to indicate you do ship quickly). But TRS+ is off the table in that scenario.
You are not TRS, don't have TRS+ listings, don't accept returns, so much of that doesn't apply, and a longer handling time might be the answer.
