OK, I'll leave Venice to someone who has been there more recently than I have but for the Rome and Florence attractions you've listed, these are the resources I'd use:
Florence:
Duomo Sky Walk (essentially the dome/terraces). This is the tour offered by the official website for the Duomo complex:
https://grandemuseodelduomo.waf.it/en/little-peek-on-florence-tour-tickets
Note that it's VERY nicely priced compared to 3rd-party tours, and does include entry (self-guided) to the other parts of the complex (bell tower, museum, baptistry, crypt). Read the details carefully: the ticket is good for 72 hours AFTER the time of your terraces visit so you may visit those other structures anytime they are open within those 72 hours.
Uffizi:
If willing to tour the museum on your own, this is the official ticketing site:
https://www.uffizi.it/en/the-uffizi
Audioguides are available in English for 6.00 €. at the entrance. Rick Steves also provides an audioguide tour which can be downloaded free to your own devices:
https://www.ricksteves.com/watch-read-listen/audio/audio-tours/italy
Note as well that if you are interested and will be in Florence long enough to take advantage, a combo ticket (PassePartout 3 Days) is available that includes the Uffizi, Pitti Palace, Boboli Gardens and National Archaeological Museum. While you must choose a day/time for the Uffizi, the others can be visited at any time they are open during the 3-day life of the ticket. https://www.uffizi.it/en/pages/combined-ticket
You didn't mention it but if wanting to visit the Accademia (same ticketing vendor as for the Uffizi):
https://webshop.b-ticket.com/webshop/webticket/eventlist?production=4
For a guided Uffizi tour (you don't really need a human guide for the Accademia) these companies get high marks from RS posters whom have used them:
Walks of Italy:
https://www.walksofitaly.com/florence-tours/
Context:
https://www.contexttravel.com/cities/florence
Others of the forum folks might have additional recommendations for guides. And onto Rome.... :O)