Are we going by the early books where recovered SL Tech was rare at best or current rule books were all recovered SL Tech was common by 3040? The Clans got as far as they did in part because of the tech gap between them and the IS. With SL Tech the Clan Invasion would have stalled sooner. With Improved Tech, it would have stalled even sooner than that.
Yes, the Clans had troops they didn't use but they didn't need them because their tech advantage allowed them to overwhelm IS forces who were shocked by what they could do. The surprise at Clan Tech wouldn't have been nearly as great if the IS had SL Tech, and even less so with Improved Tech. Add in IS units being more effective with advanced tech and the Clans would have had to use those extra forces much sooner. Even if additional Clans had joined the invasion I don't think they'd of gotten as far. They would have had to fight each other for worlds, not just for their own supply lines but for the right to attack worlds as the invasion corridor narrows. That helps the IS.
Also it only took 5-10 years for recovered SL Tech to become common. It took even less time for the IS to reverse engineer Clan Tech and expand their own weapon classes. If Improved Tech were prototyped by the IS in the 3040s it'd be common before or during the Clan Invasion. If the IS can produce Improved Tech they'd be able to produce Clan Tech once they obtain it. That means they'd be producing Clan Tech during the 3050s and 3060s. Between producing Clan Tech and their numbers the IS would be able to push the Clans back. Maybe even expel them if Comstar joined the fight.
With the Clans not invading as far, and possibly getting kicked out, the FedCom wouldn't have collapsed as Katherine wouldn't have had as much to work with. It's possible Hanse may not even had died as he wasn't under as much stress.
I think it's possible there still would have been a second Star League. I do think it'd be different though. I don't think the CC would have joined but they wouldn't have been forced to with most of the new SLDF's attention towards the Clans. The Hegemony may have been reformed in part with Victor and Omi as the First Lord and Lady. Neither would inherit their home realms to help keep the peace between DC and FC. I think WoB still would have broken off but they'd be a lot smaller. They'd still help the FWL which I think would have joined. Katherine would still scheme but she wouldn't be as ruthless as she'd know she'd be first in line for the FC. She'd prefer to be First Lord but Kurita wouldn't accept that. If the 2nd SL learns the Clan's location, they'll launch their refusal trial. If not, they'll be building up the SLDF, and rebuilding their House forces for when the Clans return or they find the Clans.
The biggest change would be with the Clans. Whether or not they say in the IS, the invading Clans would be bloodied. That opens the door to other Clans to try to settle some scores. The Society may even been able to get their tech accepted, maybe even some reforms if the IS invades.
I'm not sure if any Clans will join the 2nd SL. I do think they'll be offered a seat to keep a second invasion from happening.