Lago Maggiore

No , not a must visit. There are many towns around the lakes worthy of visits.

I stayed in Stresa a couple of years back. I found it very convenient (without a car) to travel around to different towns/villages. Beautiful area.

Canobbio (with one N and 2 B) in Switzerland I don’t know, but Cannobio (with 2 N and 1 B) is probably, in many people’s opinion, the quaintest village on Lake Maggiore. Whether that is a must or not depends on people’s taste and priorities. But if you are looking for a place to base yourself, Stresa is nice and more convenient. But definitely pay a visit to Cannobio while there.

If I don’t have a car (Avis in Stresa is currently quoting $300/day to rent a car!) how can I travel from Stresa to Cannobio Italy?

there is a bus up the west shore of the lake https://www.vcotrasporti.it/it/index.php

There are boats between Locarno and Stresa which stop at Cannobio.
https://www.navigazionelaghi.it/

This year's summer timetable is https://www.navigazionelaghi.it/doc/pdf/orari/maggiore/LocarnoArona2022.pdf
For example, the 9am boat from Locarno calls at Cannobio at 10:10 and arrives at Isola Madre at noon, 10 minutes later at Baveno and at Stresa at 12:35. Slow but exceedingly scenic.

it isn't especially easy. You could also take the train to Locarno via the Centovalli and bus or boat from there.

To link the two by public transport is one daily boat in each direction or buses which require changes and aren't especially frequent.

When I was there, pre covid, in my car it took a very long time to drive my own car between....

I would go by bus and come back by boat.
You need a SAF bus from Stresa to Intra (Embarcadero for ferry Intra Laveno). Then you switch to the VCO Bus no. 3 which goes to Brissago in Switzerland.

This is the fast bus from Stresa to Intra:
https://www.safduemila.com/wp-content/uploads/2019/03/2022.6.9-9.11-vb-arona-milano-2022.2.7.pdf
Then from Intra you take the VCO Bus 3 below:
https://www.vcotrasporti.it/userdata/bike%20sharing/orari%20estivi%202022/VERBANIA-BRISSAGO.pdf

Coming back you take the boat that leaves Cannobio at 5:05pm (17:05) which arrives back to Stresa 2 hours later.
https://www.navigazionelaghi.it/doc/pdf/orari/Maggiore/Locarno_Arona_2022.pdf

You could do the opposite, but the boat to Cannobio would put in Cannobio at 1:15pm. I think the bus going and the boat coming back gives you more time in Cannobio.
This is the boat from Stresa to Cannobio.
https://www.navigazionelaghi.it/doc/pdf/orari/Maggiore/Arona_Locarno_2022.pdf

Unless you just want a quick visit. In that case you could get there by boat at 1:15, visit a few hours and take the 5:05pm boat back. So basically no bus.

There are also local private boat tours that organize excursions. They may have some to Cannobio too. Inquire locally in Stresa. If you decide to go with the bus go on a weekday. Buses are scant to non existent on weekends.