Question Lexus RX350 - Will three carseats/boosters fit?

I am looking for advice on the purchase of two or three new car seats/high back boosters. Here's my basic info:

My car: 2007 Lexus RX350
DS1: 4.5 years old, 41.5", 35 lbs.
DS2: 12 months old, 30", 19.5 lbs.

DS1: currently rides forward facing in a Britax Boulevard CS70, manufactured in 10/2010, he has plenty of room to grow in this seat
DS2: currently rides rear facing in a Peg Perego 30/30 infant seat. The expiration is up and he's at the max height.

DH's car is a 4-door Toyota Tacoma and DS1 uses a Safety 1st Onside Air. I know it's not the best seat, but we bought it when DS1 was 2 years old so that he could use it on airplanes and during trips. It is light enough for me to carry through an airport while traveling alone with him. 95% of the time, DS1 is riding in my car in the Boulevard. DH's truck is used for occasional preschool drop-off/pick-up and weekend errands with DS1. If we go anywhere all together, we always use my car.

The dilemma/requirements for the 2-3 new carseats needed:
New carseat 1: DS2 needs to move to a rear-facing convertible.
New carseat 2: I would like to carpool to preschool with another 4 year old, approximately 40", 38lbs. His mom wants him in a harnessed seat (either a HBB or FF carseat) and I'd like DS1 to stay harnessed, also. This would require two forward facing harnessed seats and one rear facing seat in my RX350. For preschool drop off, the two FF seats must be on the passenger side and middle. I would possibly like to be able to use this extra "carpool" seat for travel for one of the kids. (If used for DS1, it would be carried through the airport, probably in our JL Childress backpack and gate checked.)
New carseat 3: Since DS1 is almost at the max height of 43" in the Safety 1st seat in DH's truck, we need another seat for this vehicle. This carseat could also be used for travel instead of the above "carpool" seat, as we can use the Safety 1st seat for DS2's travel and use either the carpool seat or DH's seat for travel for DS1.

I've considered the following options:
1. Keep DS1 in the Boulevard in my RX350. I'd like a Britax Pavilion or Peg Perego Convertible to rear-face for DS2, but I won't be able to fit any seat (that I know of) in the middle for the carpool. I tried to put a Diono Radian RXT between the Boulevard and the Pavilion and also between the Boulevard and Peg convertible, but neither combination fit across my car. Any suggestions on how to keep DS1 in the Boulevard that he is very comfortable in and put DS2 in the Pavilion or Peg convertible, plus a 5-point harness HBB or seat in the middle?
2. I could move DS2 to the currently owned Boulevard and rear-face it, then buy a new seat, possibly a Radian RXT for DS1 and either another Radian RXT or a Diono Monterey, Britax Frontier or Parkway or other harnessed HBB for the carpool. I'm not sure if any of these combinations will fit in my car due to the width.
3. I could move DS2 to the currently owned Boulevard and rear-face it, and buy two HBBs for DS1 and the carpool. This is similar to option 2., but I'm not sure which option is safer for DS1 (Radian or harnessed HBB) and which will be able to be used for the longest amount of time.
In options 1., 2., and 3., I can also buy another seat or HBB for DH's truck, or wait until DS1 actually reaches 43" and outgrows that Safety 1st seat. If it takes a long time, I can move him to a HBB instead of buying a convertible. He went through a big growth spurt a few months ago, so he could possibly stall for a while or maybe go through another growth spurt. Who knows?
Option 4.: Move the Boulevard to DH's truck and buy three new seats for my car. Again, I like the Pavillion or Peg convertible for DS2, but don't know if one of this seats, plus two more (Radians or harnessed HBBs) will fit in my car. If not, will I have to put DS2 in a rf Radian and then put two more Radians or HBBs next to it for DS1 and the carpool?

Any suggestions on how to fit the safest three seats that can be used for the longest amount of time in the back of my RX350 would be greatly appreciated! TIA!

I would definitely keep a 4yo harnessed. And with fitting 3-across trying to buckle a HBB is much harder than buckling a 5-pt harness.

Since you have 2 convertibles I would use those for your younger DS and get different seats for your 4yo (and carpool child). Are your 4yo's shoulders still below the harness slots on the Onside Air? That one has pretty low top harness slots and is most likely outgrown already even if he's under the stated max height.

Some different harnessed boosters (combo seats) to look at would be the Harmony Defender & Evenflo Securekid for narrow options for your car. Other seats to look at are the Frontier, Graco Nautilus & Recaro Performance Sport, probably none of those will work for getting 3-across. Another option for your car for the older boys is the Evenflo Titan65/Sureride, it's a convertible seat but has very high harness slots (only the Frontier is taller) and I believe is pretty narrow.

If you need to in your vehicle a Radian for your youngest may help with space. Also, Radians often puzzle well with seats on a base (like the Blvd) when they're facing the same direction.

Thanks so much for your fast response, Ann. I'll look into those combo seat suggestions. I forgot to mention that my older son still occasionally falls asleep in the car, so I need a seat that will allow him to comfortably rest his head on the side. I'm not sure if that changes any of your advice.

Also, of your suggestions, which seats are the safest? If not for the carpool, I'd keep him in his boulevard and just buy a new convertible for my younger son. If the combos or other seats are not going to offer the same amount of SIP or comfort, I'll forgo the carpool. While the carpool would be convenient, I don't want to compromise my son's safety.

Also, thanks for the heads up on the Onside Air height limitation. I'm going to check the strap height tomorrow. I rarely buckle him into that seat since he rides with my husband in that car and I didn't realize that he might have outgrown it before reaching the max height.

Thanks again so much for your help!

There is no safety difference in a 5-pt harness between a FF convertible and using the harness in a combo seat. And since all seats need to pass the same testing they're all safe to use, there's no way to say one is safer than another. (even if some companies will tell you that)

For comfort sleeping, I know some kids have head-slump in the Nautilus. My DS had some problems when we first started using our Frontier but once he figured out how he needed to position himself he was fine.

Some notes for installing 3-seats in a row...
It usually works best to install all of the seats with seatbelts, that often allows you to slide the outboard seats closer to the doors so there's more room in the middle. You also want to make sure that all seats are installed independently (not relying on the pressure of the seat next to it to keep it tight). The easiest way to check is to install the center seat tightly first, then install the outer seats tightly. Then remove the center seat and check how tight the outer seats still are. If they have more than 1" of movement at the belt pat than it's not an acceptable independent install for 3-across.