Hello ledkr -- my DD2 is almost exactly the same age as yours (I was in your AN EDD group under another name) and has mixed hearing loss (profound sensorineural in one ear, possible mild-moderate permanent conductive in the other ear and on-and-off (but mostly on) glue ear since birth layered over the top of those - she has bilateral BTE aids at the moment) and a significant language delay so I've been in a LOT of audiology appointments
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Firstly, waiting three months after first getting a flat tympanometry reading is perfectly standard because it does fluctuate a lot. And the research suggests that in the majority of cases "watchful waiting" while using hearing aids will have better long-term results than grommets even if the glue ear doesn't improve.
I'd potentially be concerned about the "slight low level hearing loss", to be honest, given that it's combined with a speech delay -- did they ever tell you what level that was? Even a very slight loss can have a big impact when acquiring speech. Did they do a full hearing test today or just the tympanometry? In your position I'd probably push the question of hearing aids at your next appointment if they don't. It's good that you're already in the SLT system
It's worth registering with the NDCS, as well -- they have some very useful publications, including a good leaflet on glue ear, and a helpful parents' forum.
And do the standard hearing-impaired stuff: keep background noise to a minimum, don't mumble, don't cover your mouth while talking, don't stand so that you are backlit while talking, make sure you have DD's attention before you try to convey any important information, check that she's understood what you are saying after you've said it.