Hi and welcome!!!
Please put our minds at ease that your limo-style sedan is just bigger and plushier, but has the regular front-of-the-car facing seating. (You cannot install carseats on vehicle seats facing the rear or side of the car.)
Please also post your kids' weights and heights - this will help us make better seat reccommendations. Depending on your answer, we may even recommend that you give the Radian to the baby and buy a different FF seat for the 3.5yo.
The Radian is a seat which
allows you to tether when installed RF - the RF tether is by no means required for a safe install. In fact, some people are not comfortable RF tethering because of the lack of designated anchors in most vehicles. The other manufacturer allowing (not requiring) RF tethering at this time is Britax.
Top tethers for
front facing harnessed carseats are
required at all times in Canada. If your vehicle does not have a top tether anchor available, you must look into getting one retrofitted. In the meantime, you are using your FF seat illegally and also reducing the safety of the seat in a crash. If your child fits the Radian RF by height and weight, then RF would be your best option until you can retrofit the tether anchor.
Latch, ummm, let's speak Canadian.... Lower UAS anchors are not able to be retrofitted in the majority of vehicles (I think VW has a parts kit for some of its vehicles?). However, all seats sold in Canada MUST be able to be installed with a seatbelt even if they are equipped with UAS hardware as well. We can help you figure out a seatbelt install if you are finding it confusing beyond the top tether issue. So, you really get to take your pick of any seat you want, and that will install well in your vehicle - it is sometimes an individual fit.
Since you say that your 14mo is tall, and most seats are outgrown by height before weight by most kids, I would not look at the Graco MyRide as a first option. It has a shorter shell than other seats available (outgrown faster RF) and shorter top slots (outgrown faster FF). The Britax Advocate has about the same stats as the MyRide, but also RF to only 35lbs as opposed to 40lbs for the MR. Depending on your DD's stats, the RF weight limit may or may not make a big difference to your "better" seat choices. The other Britax convertibles except the Roundabout50, have a RF weight limit of only 30lbs, which unless your DD is very light I would steer away from. I am trying to remember if your vehicle would have locking seatbelts but I can never remember the years when things change over, etc. If you do not have locking seatbelts for some reason, then the Advocate has the advantage of built-in lockoffs, saving you some hassle. All these seats (Britax convertibles and the MR) have a 6year lifespan before they expire.
The seats I WOULD look at first because of the very tall shells and top harness positions, are the Radian (which you can try out to see if a RF install is possible; 40/45lbsRF/65lbsFF/8yr exp), the Safety 1st Complete Air (40lbsRF/50lbsFF/8yr exp) and the First Years True Fit (35lbsRF/65lbsFF/7yr exp). The True Fit also has built-in lockoffs.