This is more unbalancing than you think
I've played a WM Sorcerer a couple times and it is pretty fun. I relied upon some guides to build a character that was optimized and in general, WM works best away from the party. In general, one of the restrictions on WM is that you cannot reliably use it outside of combat and this proposal would change that. One of the key things to take away from the surge table is the following (from Cognomen's Cassowary's guide):
There it is, the entire table broken down for easy digestion. To tally, there are seven gold ratings (14%), things you are happy to have virtually any time they come up. There are eight light blues (16%), signifying effects that are less powerful but you're still happy to have. There are six solid blues (12%), mostly ancillary effects and oddities which are on the whole positive. 42% of the table is definitively positive.
There are two green results (4%) which are slightly positive, nine (18%) which are RP centric, and three (6%) which depend on how the DM plays them. (Two of them summon creatures within five feet of you, so you might actually be able to use them for cover.) 28% of the table runs neutral to slightly positive.
Three purples (6%) represent results which are negative on balance but might have some useful attributes, and three reds (6%) comprise the worst, outcomes with no redeeming value. 12% negative effects move the total to 82%. What about the other 18%?
What remains are the situationally good or bad results, six of which (12%) are better if you're near enemies and three (6%) near allies (including self-centered fog cloud, which is something of an edge case). This is important for understanding how to optimize your play. If you are near (for most effects, within thirty feet of) your enemies but not your allies, you push the number of positive outcomes up to 54%, while increasing the negative to only 18%.
Outside of combat, your proposed house rule would permit players to effectively abuse cantrips to force a surge whenever they want. In general, this will most likely be good for the player; the exact nature of how it's good will be variable, but there are several things on the table that can come up which permit a sorcerer to fully recover their resources:
- For the next minute, you regain 5 hit points at the start of each of your turns
- You regain 2d10 hit poins
- You regain your lowest level expended spell slot
- You regain all expended sorcery points
This is only 4 out of 50 possibilities on the table, but a lot of the table possibilities aren't negative provided the party gives the WM Sorcerer space, so more often than not the party will come out ahead. As this house rule let's them persistently try indefinitely, there's nothing but notably bad luck preventing them from fire bolting a wall until they manage to recover all their hit points/spell slots/sorcery points.
This would substantially worsen when the WM Sorcerer reaches 14th and gains Controlled Chaos. Since you're doing LMoP, that's a bit out of reach, but the impact on the long game is worth noting.
The built-in means to deal with the issue of infrequent surges is the Tides of Chaos class feature. As a whole, WM Sorcerers fully come on-line at 2nd level when they gain that feature.
The querent has stated they are the DM and would indicate when they want surges to occur, but I would recommend taking an opposite tact. Indicate when you don't want them to occur because you will forget and it will be annoying from the player's perspective every time you do. As a DM, you've too much to keep track of as is and every time you forget to ask for a surge is a chance that player doesn't get to use their class feature that every other sorcerer origin gets to enjoy by default. Let the player keep track of when to roll for surges on their own, including with Tides of Chaos recharge and you will find that surges happen pretty regularly, which is what should happen.
If after seeing the results of play through 5 levels by letting the player control their surges, I think you'll find the surges happen more than enough on their own.