I'm honestly not sure if this sounds entirely reasonable or like a potential problem. I think the devil is in the details, and your question isn't as detailed as it could be ( though it is detailed! )
Reasonable-
If everyone is working PT and equal time commitments, than this sounds fair.
If everyone's money is treated as a convertible note, than this sounds fair.
Not Reasonable-
This is where some of the (lack of) details worry me.
There's the line "[high salary] which probably can't be met by the start-up anytime in the next few years". Stuff like this always worries me, because people often start saying "I'm making 250-500k a year now, so I want my compensation to be based off that". The problem with that line of thought with FOUNDERS is that startup co-founders ( and essentially all company owners ) are somewhat expected to work at scale and draw as little as possible from salary; their primary compensation comes from profits and added value.
It's one thing for an EMPLOYEE to value their contribution and start negotiating for more equity because of the loss of salary. They're typically looking at something < 1% equity and half their salary; it's a terrible deal. Employees , even employee-owners , are thinking about themselves first , the company second. Their goal is to get as much as possible in the short term, as they have no long-term control.
With a cofounder though, there are the competing goals of self-preservation and whats-best-for-the-company in the long term. If their salary is 500k , the company can only pay cofounders 150k for a few years, and this is a huge issue for them -- that raises a red flag for me . I'm worrying why they're so focused on the short-term and "getting things out" of the company. I'm worried why they think cash should go into the salary instead of into more employees/marketing. I'm also worried about why they're treating this as a side-step or promotion in their corporate life, instead of redeveloping their career ( It's really common for people to switch careers and make half as much (or less) for more fulfilling or potentially lucrative work ). They're auditioning for a job-role that pays $x; you shouldn't be auditioning for an employee that is currently making $y.
It's also another issue where someone wants compensation for their time at a different rate than others. Some people will say "I make 500k a year, you make 100k, so my PT hourly contribution is 5x yours".
I don't know if any of these things apply, but I'd watch out for them.
Anyways, in your situation I'd want to treat any of the cash-in from co-founders as a convertible note with some preferential terms on the discount rate (ie, do better what you'd offer in a seed round ).
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