The length of your summer sounds similar to mine. I hope you will keep us updated on your approach and which pieces you decide to try. I like the idea of SF's list and may have to come up with a similar version.

@ Lisap - the counting exercise was an eye opener. I’ve been investing in clothes I wear 2/3 of the year, and in some cases 2-3 months, but basically treating 4 months (mid May to mid September) as an afterthought.

I don’t think every summer is quite that hot here, actually. But I guess my perception is somewhat skewed by going away to the cottage where it’s generally cooler, as well as working in an office with A/C normally. Still, I can remember cooler, rainy summers in the past decade. Always a few hot days though.
When looking for light weight tops consider whether they are gauzy, sheer or eyelet and will require an under layer and what that might be - a lot of linen tees are pretty sheer, for example. I find this to be a fatal flaw with so many summer tops that should be breezy. I don’t find camis, even wicking under layers like Uniqlo Airism, to be cooling.

Yeah, southern Ontario is hot, if you don't have AC. Not as bad on the lake as inland, but even so...

You may have a microclimate where you are, too -- it might be hot if you are in the garden, for instance, and less hot nearer the water.

Kingston was a bit cooler than where you are, but we still got at least 2 months of hot every year. And humid.

To go back to the purchase plan...I'd start the footwear hunt in March. You'll be able to find sandals online starting then, maybe even February, but for me it was always too early to think of it, then.

Clothing search can start in thrift stores between March - June -- depending on when they put stuff out. Hard to say when high summer items will appear online at retail.

You have done really well with summer dresses from thrift in your new town, right? You got the white one and the red print one, and from your WIWs, you wore both of those quite a lot. I do find that many summer dresses are "done" in two years rather than three, especially if worn a great deal. In years when I've only owned two, they lasted 2 years. In years where I owned 3 or 4, they lasted much longer. My current summer dresses will all probably do me for another season or two, but I don't wear them as often now that I live in a more temperate place.

I got a pair of these Icebreaker shorts for hikes last year, and holy moly, I really love them and they look nice enough for casual outings! They're much lighter weight and cooler than denim.

Yes, Suz, summer clothes take a beating (more frequent laundering) and don’t last as long. I still have the white dress, but the red one was retired after 2 summers. Not bad for a $5 thrifted dress.

I like the idea of switching into an easy dress for the pm, after the post gardening wash-up.

Daytime temps 30deg.C ?? I have a tiny capsule for that, just long enough to survive with one load of laundry a week for the two week heat spells we occasionally have in our summers :). Were I to create one that needn't be work appropriate, I'd probably go for:
2 sleeveless dresses
2 pairs of linen ankle pants
2 pairs of cotton chino bermudas
1 pair of denim shorts
3 basic tank tops
3 printed sleeveless tops
1 sleeveless button-down
1 chambray or lightweight cotton shirt for layering
1 lightweight cardigan
2 denim jackets (1 white, 1 blue)
1 pair of sporty sandals or Birkenstocks
1 pair of flat gladiator sandals
1 pair of cotton sneakers (Converse type)
(All the toppers in the assumption that mornings and evenings are cooler.)
Of course if my entire summer was that hot and I'd wear almost only shorts like you I'd probably get 7 pairs, and a stylish playsuit to mix things up.

RunCarla, what a great time to plan - but stay flexible as you plan! - for high Summer

I LOVE Summer dressing. Personally I stick to breezy dresses, skirts, cropped white jeans, and cute Summer pants with shirts and blouses. All roomy and made of natural fibres. Straw hat and prescription sunnies are vital. Sneakers because I barely wear sandals. Sometimes I bust them out - like I did in Italy. And a slew of denim jackets for when it's breezy.

I get very high heat and know how to dress for it. It's how I grew up! I like to cover up in the heat, which might differ to you. I don't wear shorts unless I'm swimming with Sam, or at the beach in a cozzie.

Lots of great ideas from the others. Hope you're inspired.

@ Angie - I am! I promise to stay flexible.

Thank you

My summer workhorse are denim skirts, dark denim and white, a “jean jacket” made of white handkerchief linen, a tee shirt dress, Capri or ankle pants, especially in linen. Silk button down shirts used as light toppers. I like natural undyed linen and breezy sleeveless tops. I never go out without sunglasses and/or a brimmed sun hat. Looking at what you still have from last summer, it looks like you have a good start on a red, white and blue/navy color scheme.

If I were making a summer wardrobe “from scratch”, I’d start by listing the types of activities I’d like outfits to cover, then define each activity proportionally according to how many outfits I’d likely need. (This could mean three concerts need three outfits, and three hikes only need one.)
My activities would not take covid quarantine into account, because we all hope that will be less by summer. For example:

1. Most of day at home, chores, hobbies, yard work, walks. 4-5 days/week. Shorts, sleeveless knit tops, sneakers or sandals.

2. Most of day in town, shopping, errands, coffee with friend, art group at senior center, appointments. 1-2 day/week. Shorts, casual skirts, sleeveless blouses, light topper, sandals, sneakers, jewelry accessories, bags.