In no particular order: "ballpark cost on a regular single story 1980's build 1,800 sq. ft." means little for what we do. The measurement is on the "up & over", not the straight line of the concrete slab, etc.
The best thing is for you to check your PM & I'll have an offline chat with you regarding a lot of topics, related.
Best time to replace, you are definitely onto something. Obv. hail is random & it can show up in random places & times. We cannot predict much other than Spring has a higher possibility. I have plenty of customers who want to wait until "season" is over so they don't have the probability of another strike, so... yeah. & You are right about "dusting", where shingles don't bond due to colder temperatures & high wind can cause small bits of dirt & whatnot to get between the shingle courses. As to the heat, we work 365 days a year if the conditions are right & even in the 100 degree summer days, we take breaks as needed.
Supply chain, we are actually doing a decent amount of Home Depot purchases & if it's a ground drop situation where we have to hand load anyhow (cannot load via boom truck due to trees, power or other lines) then a "wholesaler" isn't a possibility anyhow. Also, HD is often lower priced on the same products that we find through the "wholesaler" (i.e. mostly only roofing products). There ARE some specialty shingles that cannot come from HD (recently did roof for @joe dirt & his neighbor to the R - Pflugerville - we used a specific shingle that cannot come through HD unless a stupid high markup is added on. The delay on the Atlas shingle was maybe 3 weeks IIRC & the current HD products is maybe 7 days on the longest. For Central Texas Home Depot, it's Owens Corning & for Architectural / Dimensional shingles (30 year type shingles) with the Oakridge & Duration (Duration is a step above in the Dimensional line... thicker nail line, double cement line, etc). Both are appropriate to use.
Depending on your various structure's particulars & product needs / wants, if we got a deposit today, the roof could go on inside of 10 days if needed.
Check your PM's.