Commuter Flights within EU?

It might be cheaper. I use skyscanner.com to look for intra-European flights, but I've never needed to get separate tickets for my transatlantic flights.

One additional issue, besides the need to handle your own luggage at the transfer point and the possibility of being subject to a second country's COVID rules, is that all the risk of a delayed or canceled initial flight falls on you. If you have a through-ticket, your airline is responsible for getting you to your final destination. If you have two tickets and the first flight doesn't get you to the transfer point in time to catch the second flight, you will be buying a replacement airline ticket at last-minute prices (or taking a possibly-long train trip, likely also not cheap). This isn't a negligible risk at the best of times, and it's a lot riskier now, with airlines changing a lot of schedules and canceling flights.

I gather you're trying to get to Venice. Have you looked at flying into Milan? It's often reasonably inexpensive for me, but fares are highly origin-dependent as well as destination-dependent. If Milan doesn't work, what about Rome? Milano's Central Station is about 2-1/2 hours from Venice. Rome's Termini Station is 4 hours from Venice. You'd need to add time to get from the airport to the downtown train station (faster in Rome than in Milan, I believe), but it's still a doable trip if you have a reasonably early landing time in Milan/Rome. Both Trenitalia and ItaloTreno run super-fast trains from both Milan and Rome to Venice. Trenitalia alone has hourly departures during the busiest part of the day.

Unfortunately, you have the same scheduling uncertainty with a train you have with a flight: Who knows when your flight would actually land in Rome or Milan, you'd get through Immigration and reach the departure train station. Buying the train ticket after arrival would probably mean paying the full maximum price for the ticket (or close to it) rather than snagging a bargain fare. The ticket from Milan would be less costly than the ticket from Rome.

One possibility would be to gamble on a reasonably-on-time arrival and buy one of those bargain-priced tickets way in advance--maybe for 4 or 5 hours after your flight is supposed to touch down. (I'm not the best one to give guidance on that timing.) You might figure the savings from getting the cheap ticket would justify the risk that you'd be delayed enough to buy a replacement ticket. People take that gamble pretty often, and it often works out.

A couple of other things:

The baggage limits for flights within Europe are often more stringent than for transatlantic flights. You could easily end up having to pay a checked-bag fee (can be as much as about $50, I think, for a not-huge bag) for a suitcase you could carry on in the US. And some airlines have extremely light weight limits (8 kg!) for a carry-on bag; that can trip you up even if you do have a legitimate 22" carry-on. Definitely Google for information on the potential Europeans' baggage rules to be sure you're working with complete cost information for comparison purposes.

You'd also probably have to pay a seat-selection fee if you want to be sure of sitting next to you travel companion. That might or might not be more than a US airline would charge. I've been able to avoid transatlantic seat-selection fees, but it's harder on the European airlines.

If you decide to fly the final leg, pay careful attention to both airports used by your airline. RyanAir flies into the Treviso airport rather than Venice-Marco Polo. Treviso is by all accounts a nice city, but you'll probably be anxious to get to Venice. It also uses the Bergamo airport rather than Malpensa or Linate in Milan. Again--a very nice city, totally worthy of a visit, but you're trying to get to Venice. Also be sure not to fall for the nightmare scenario of landing at London-Heathrow and having a flight out of Gatwick, Luton or Stansted.

The big issue in lost of the second ticket should you first flight be delayed. The second airline has no obligation to you to honor the ticket if you miss the flight. And, I doubt if it is cheaper.

Would it be cheaper to switch to a EU commuter instead of staying with the international carrier for the last leg to Venice

Possibly, but not a certainty or probability. And doing separate bookings means you assume all the ri$k of a missed connection

If you do try a DIY separate itinerary, allow at least 4 hours between flights to give yourself a fighting chance of success.

I don't think I've heard "commuter" used to reference a flight or airline since the 80s. Rather than the polite, private businesses of yesteryear's regional turbo-prop carriers, you'd probably be looking at lean, efficient, inflexible Low-Cost Carriers. If there are actually savings it would have to be weighed against both the risk and hassle.

If you have an extra day or two, you can turn it into a "stopover". Depending on the city, it could be a fun chance to sample another European city and get over jet lag before your onward journey.

If you do this, I’d suggest you plan to spend at least one night in your arrival city before flying to Venice, On separate tickets, your connection is not protected if your inbound flight is delayed or canceled. Given that airlines are canceling and changing reservations frequently these days, separate tickets are riskier than usual.