So, basically a loan, but you risk investing 30M on a player that maybe doesn't work for you, while with a loan the player goes back and you spent 0$
0 interest, your "interest" is basically the salary you pay him, best case scenario you enjoy a good LB for 2 years and break even, now you need to look for another LB in 2022 when the market will probably be higher. There's a reason why "feeder" clubs go for these buyback options while big teams don't, and it's because when you sign the right player you want to keep him, if you want to push for titles you can't be looking for replacements every two years.
If you're paying 30+5 you're not paying below market rate, Real sold Hakimi for 40M without buyback, both of them have 2 whole seasons at elite level but Hakimi is 2 years younger. Your deal is way worse than the one Inter got, if the transfers end up being bad, Inter will at least have a younger player (by the time your buyback ends, Hakimi will be the same age as Reguilon today) to try to get rid of, if the transfer goes good, Inter at looking at either keeping a world class full back or a big payday, not just recoup two years of salary.
And being almost 24 y/o you're not paying for "promising" either, you pay for what you saw at Sevilla, that's probably what you'll get, which is probably good enough to be top 20 in the world, you're not signing Alphonso Davies' potential, he's not going to turn into prime Marcelo or current Robertson in the next 2 years, and the kicker, even if he did, you'll lose him.
Not all, but you can have it better, like Inter.
You just need to focus on one thing, Real Madrid don't really have a backup to Carvajal (28) since Odriozola's time is done with them yet they let go of one player they already had that could potentially challenge the spot without a buyback clause. Their backup to Mendy is Marcelo, who will leave in the next two years. So, tell me, do you think you got anything closer to a good deal when you just solved their LB situation for probably the next 8 years while you pay 30M on a tight market window for a 2 year patch?.