I think you could TRIM it in another machine, yes. That process should just look at the filesystem and TRIM all unallocated sectors, so any OS that understands FAT32 should be able to do that.
The biggest thing that leads to massively increased wear (and slowdown) on an SSD is "write amplification". This is caused by the drive's internal controller having to shuffle data around in the Flash cells in order to accommodate the next write operation without messing up existing data. All that shuffling multiplies the number of writes that are performed far beyond what is apparent to the user. It can get into the 20x and higher range.
This happens as the drive approaches being "full". I put that in quotes because the firmware doesn't understand what logical addresses (LBAs) of the disk are actually being used. Once an LBA has been written to, the drive will believe forever that LBA refers to valid data. Win98's lack of TRIM means it will never be told that LBA is no longer in use. So it will keep preserving it, even after Windows' filesystem has forgotten about it.
Over time, you can approach the point where nearly every LBA on the disk has been the target of some write operation. The drive will dutifully try to preserve all of it, and as the remaining number of unused LBAs gets very small, the drive's perception of it's free space also gets small. Write amplification becomes significant. Performance falls off and wear accelerates.
In this situation, Windows may report the drive having a lot of "free" space, but the drive doesn't understand that. If an address has been used once, the drive still thinks it's important.
To prevent that, don't let every LBA get used. You can do that with partitioning:
First use some means to inform the drive that every LBA is free, so that it will stop maintaining every piece of data that it currently thinks is important.
There are various ways of effectively TRIMming the whole drive. Some ways are more elegant, but a simple and universal way to accomplish this is to do a "secure erase". The downside of "secure erase" is that it doesn't just TRIM the drive, it actually does an exhaustive overwrite of every cell, which shouldn't be necessary for our purposes.
Once the drive is completely TRIMmed, then set up your partition(s). Don't allocate the partitions under Win98. Instead, use a modern version of gParted or similar and enable the option to align the partitions on 1MB boundaries. This resolves the partition alignment issue that can otherwise occur when using old versions of Windows.
It's fine to use Win98 to do the formatting, just don't use it to make the partitions.
Don't partition the entire drive. If you leave say 25% of the drive unpartitioned, this means that Win98 will never direct a single write operation to any of those LBAs. This means that no matter what, the drive will never reach the point of thinking it has less than 25% free space. At 25% free space, write amplification shouldn't be much of a problem.
By leaving unpartitioned space, you are basically tricking the drive into having more reserved Flash storage than the manufacturer originally provided. This is also one of the differences between consumer and "enterprise" SSDs. The latter have more reserved Flash storage so that they are less susceptible to this issue, and because they are sometimes used in applications where TRIM isn't as feasible. This trick makes the drive harder to beat down.
I recall some article(s) that studied this at the big review sites (Anandtech or similar). This was an issue when SSDs were new and TRIM wasn't yet supported. This is really the same situation we're in if we use an SSD on old OSes like Win98, so for us those articles are still relevant.