First off, glad to hear you're doing great and I hope the trials lead to success!
There is a lot to unpack here but I'll give it a shot. Before I get going too far, I see that you have
Hilton points already. Are you interested in continuing with them? I only mention because the Hilton cards have a high points welcome offer going on now with a free night certificate.
The offers end 7/19 so I'd look at the Biz card today (7/18) and decide if you could use the offer. The business option won't report to your CR and there's no crazy eligibility charts like Marriott so getting it won't really affect much of anything else on a future CC path. But if they're not your favorite choice then trying to find somewhere to use the FNC in the next year not really helpful.
Don't just FOMO apply because everyone else is if it won't bring you joy They've done similar bonuses before so I'm sure it'll probably come around again at some point. But I'd feel bad if I didn't point it out and it'd be useful for you. (I'll also point out if you decide on a personal Hilton flavor it'll make a Chase card journey a little harder, so if you're interested in any of Chase's cards I'd focus on the business option unless you have a really good case for another. A personal would change the degree of difficulty on the Chase cards.)
Secondly, let me make sure I understand your cards in the last 2 years. Did you really cancel a card 3 months after getting it? Was there a reason to do that? As a general practice we recommend holding a card for a year. For some bonuses this is a requirement to avoid clawback of the bonus under the agreement terms, for some if you've paid an annual fee may as well get use out of it since you won't get a pro-rated refund anyway ( except residents of MA usually), and then mostly it's just good form to encourage continued promos from the banks. But sometimes things get in the way and it'd be good to understand that.
You look like you are 4/24 so Chase cards are available. But with the AU card as soon as you get another on your credit report you're locked out until Oct. You'd be able to try recon to overlook that card in the calculation but it's not a guarantee that they will. We advise avoiding AU cards for this reason. There are ways to share the spending without having an AU card. If most places are modern you could add it to a digital wallet to use there. Can also request a replacement card because it was "damaged" and the copy should work same as original as long as no one asks for matching ID. (Don't pick 'lost' as an option, they will give you a new card number and lock the other, defeating the purpose.) For Chase, most times if you ask for a card to be expedited, you get two - a copy they rushed and the original request. That comes in handy.
Your exception would be employee cards on biz accounts. They won't report to your CR so no worry about adding to your 5/24 count.
P2 is in an interesting place where he's 4/24 or 6/24 depending on those store cards. For Tractor Supply, is that a store only card? That might not count towards Chase's calculation depending on how it's listed on the report. And the Lowe's card, likewise, is that a store only line? And if it's commercial, is it really showing up on his personal CR? Otherwise if interested in Chase you could try to recon for both the store and AU cards if they lead to a denial.
Third let's better understand your medical travel and how we can best use that for vacay travel. Does the clinical trial work on a reimbursement basis so you pay for the flight and they reimburse your ticket costs? Or is there a specific way that needs to be booked? If the former then you have some good flexibility in using a credit card travel benefits.
I was counting your cards for Chase as I think generally the CSR is recommended for most flexible travel protections for trip delay and trip interruption. Some cards require that the full fare be charged to the card to be covered but the Sapphires allow a points booking as long as some portion of the fare is charged to the card. So that would be handy for both your routine, reimbursed travel and leisure travel from points. AmEx also has coverage but, IIRC, you have to book a round-trip flight and have it fully charged to the card.
I was also thinking of the CSR if you're reimbursed as the $300 travel credit takes a nice chunk out of the AF.
UR points could be transferred for flights on Southwest or United, which can help with your companion expenses or build up for either of your anticipated trips.
Actually, I still haven't formulated much if a path yet but are your Radisson points the Americas or Europe program? The Americas are converting to Choice points today which would be more helpful for the West trip than Europe. And vice versa.
With your monthly spend it doesn't seem like you'll have a problem hitting nearly any bonus so that's helpful. Since you're open to business cards, have you looked at the Chase Inks? That'll help accumulate UR points while you're under 5/24. If so, then we can work some timelines to get both that and some personals going.
Sorry wasn't as much help off the bat! I am a Chase UR fan for their flexibility but I don't just want to blindly recommend that path if there's something else better for you. I'm still mulling it over, sorry!